As i wanted to test few thing related to asm diskgroup on my test box, had to drop the diskgroup. Before dropping the diskgroup had dropped the database using RMAN.

A S M    D I S K G R O U P    S P A C E   U S A G E   R E P O R T
                                                                                                                    Required
                                                           AU                                          Hot     Cold     Free   Usable
                                   ASM        RDBMS      Size  Offline    Total     Free     Used     Used     Used   Mirror     Free
DiskGroup       STATE       Type   Compat     Compat     (MB)    Disks     (GB)     (GB)     (GB)     (GB)     (GB)     (GB)     (GB)
--------------- ----------- ------ ---------- ---------- ---- -------- -------- -------- -------- -------- -------- -------- --------
DATA02          MOUNTED     EXTERN 10.1.0.0.0 10.1.0.0.0    1        0        2        0        2        0        2        0        0
DATA            MOUNTED     EXTERN 11.2.0.0.0 10.1.0.0.0    1        0        4        3        1        0        1        0        3
                                                                       -------- -------- -------- -------- -------- -------- --------
Total                                                                         6        3        3        0        3        0        3
SQL>  drop diskgroup DATA including contents;
 drop diskgroup DATA including contents
*
ERROR at line 1:
ORA-15039: diskgroup not dropped
ORA-15027: active use of diskgroup "DATA" precludes its dismount

Using v$asm_client checked if any database client is still on

SQL> select * from v$asm_client;
no rows selected

Checked for the spfile location of the ASM instance and it resided in +DATA diskgroup, leading to ORA-15027

SQL> show parameter spfile
NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
spfile                               string      +DATA/asm/asmparameterfile/reg
                                                 istry.253.749745881
SQL>

To resolve and drop the diskgroup did the following
1. create pfile in another location –

SQL> create pfile='$ORACLE_HOME/dbs/init+ASM.ora' from spfile='+DATA/asm/asmparameterfile/registry.253.749745881';
File created.

2. Shutdown and start the asm instance using PFILE

SQL> startup pfile=$ORACLE_HOME/dbs/init+ASM.ora
ASM instance started
Total System Global Area  284565504 bytes
Fixed Size                  1343692 bytes
Variable Size             258055988 bytes
ASM Cache                  25165824 bytes
ASM diskgroups mounted
SQL> show parameter disk
NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
asm_diskgroups                       string      DATA02, DATA

3. drop the diskgroup

A S M    D I S K G R O U P    S P A C E   U S A G E   R E P O R T
                                                                                                                    Required
                                                           AU                                          Hot     Cold     Free   Usable
                                   ASM        RDBMS      Size  Offline    Total     Free     Used     Used     Used   Mirror     Free
DiskGroup       STATE       Type   Compat     Compat     (MB)    Disks     (GB)     (GB)     (GB)     (GB)     (GB)     (GB)     (GB)
--------------- ----------- ------ ---------- ---------- ---- -------- -------- -------- -------- -------- -------- -------- --------
DATA02          MOUNTED     EXTERN 10.1.0.0.0 10.1.0.0.0    1        0        2        2        0        0        0        0        2
DATA            MOUNTED     EXTERN 11.2.0.0.0 10.1.0.0.0    1        0        4        4        0        0        0        0        4
                                                                       -------- -------- -------- -------- -------- -------- --------
Total                                                                         6        6        0        0        0        0        6
SQL> drop diskgroup DATA including contents;
Diskgroup dropped.

The other way of dropping the diskgroup is –

A S M    D I S K G R O U P    S P A C E   U S A G E   R E P O R T
                                                                                                                    Required
                                                           AU                                          Hot     Cold     Free   Usable
                                   ASM        RDBMS      Size  Offline    Total     Free     Used     Used     Used   Mirror     Free
DiskGroup       STATE       Type   Compat     Compat     (MB)    Disks     (GB)     (GB)     (GB)     (GB)     (GB)     (GB)     (GB)
--------------- ----------- ------ ---------- ---------- ---- -------- -------- -------- -------- -------- -------- -------- --------
DATA02          MOUNTED     EXTERN 10.1.0.0.0 10.1.0.0.0    1        0        2        2        0        0        0        0        2
                                                                       -------- -------- -------- -------- -------- -------- --------
Total                                                                         2        2        0        0        0        0        2
SQL> create spfile='+DATA02' from pfile='$ORACLE_HOME/dbs/init+ASM.ora';
create spfile='+DATA02' from pfile='$ORACLE_HOME/dbs/init+ASM.ora'
*
ERROR at line 1:
ORA-17502: ksfdcre:4 Failed to create file +DATA02
ORA-15221: ASM operation requires compatible.asm of 11.2.0.0.0 or higher
SQL> alter diskgroup DATA02 set attribute 'compatible.asm'='11.2.0.0.0';
Diskgroup altered.
SQL> create spfile='+DATA02' from pfile='$ORACLE_HOME/dbs/init+ASM.ora';
File created.

Shutdown and started the ASM instance using the SPFILE which resides on +DATA02 now and is the only asm diskgroup associated with the instance. When we try to drop the diskgroup we get the same ORA-15027

SQL> drop diskgroup DATA02 including contents;
drop diskgroup DATA02 including contents
*
ERROR at line 1:
ORA-15039: diskgroup not dropped
ORA-15027: active use of diskgroup "DATA02" precludes its dismount
SQL> alter diskgroup DATA02  dismount force;
Diskgroup altered.
A S M    D I S K G R O U P    S P A C E   U S A G E   R E P O R T
                                                                                                                    Required
                                                           AU                                          Hot     Cold     Free   Usable
                                   ASM        RDBMS      Size  Offline    Total     Free     Used     Used     Used   Mirror     Free
DiskGroup       STATE       Type   Compat     Compat     (MB)    Disks     (GB)     (GB)     (GB)     (GB)     (GB)     (GB)     (GB)
--------------- ----------- ------ ---------- ---------- ---- -------- -------- -------- -------- -------- -------- -------- --------
DATA02          DISMOUNTED         0.0.0.0.0  0.0.0.0.0     0        0        0        0        0        0        0        0        0
                                                                       -------- -------- -------- -------- -------- -------- --------
Total                                                                         0        0        0        0        0        0        0
SQL> drop diskgroup DATA02 force including contents;
Diskgroup dropped.