Drop an unmounted ASM diskgroup

We are getting our hands dirty with ASM and will be moving all our databases from RAW devices to ASM in the next couple of months. My worst fear came to be when a Unix Sys Admin used a disk already allocated to ASM for a local filesystem (luckily this did not happen on a production server!).
When ASM realised this is immediatly issued a ALTER DISKGROUP PCASDG1 DISMOUNT FORCE command.
By the time the Unix Sys Admin returned the disk it was unrecognozable by ASM as the disk headers had been overwritten. The diskgroup was unrecoverable and unusable.

SQL> select GROUP_NUMBER, NAME, state from v$asm_diskgroup;
GROUP_NUMBER NAME STATE
------------ --------- ----------
0 PCASDG1 DISMOUNTED
2 PCASDGF MOUNTED
3 PCASDGR MOUNTED
3 rows selected.

SQL>alter diskgroup PCASDG1 mount;
alter diskgroup PCASDG1 mount
*
ERROR at line 1:
ORA-15032: not all alterations performed
ORA-15040: diskgroup is incomplete
ORA-15042: ASM disk "19" is missing

SQL>drop diskgroup PCASDG1 including contents;
drop diskgroup PCASDG1 including contents
*
ERROR at line 1:
ORA-15039: diskgroup not dropped
ORA-15001: diskgroup "PCASDG1" does not exist or is not mounted

SQL>alter diskgroup PCASDG1 check;
alter diskgroup PCASDG1 check
*
ERROR at line 1:
ORA-15032: not all alterations performed
ORA-15001: diskgroup "PCASDG1" does not exist or is not mounted

I had to recreate the whole diskgroup from scratch. I followed Metalink Note 387103.1 to recreate the diskgroup.
Basically you clean up the asm disk header of all the disks in the diskgroup. This will also remove the whole diskgroup from asm.

SQL> select 'dd if=/dev/zero of='PATH' bs=8192 count=100' from v$asm_disk where GROUP_NUMBER=0;

$dd if=/dev/zero of=/dev/oracle/dg/c11t15d2 bs=8192 count=100
100+0 records in
100+0 records out

$dd if=/dev/zero of=/dev/oracle/dg/c11t7d0 bs=8192 count=100
100+0 records in
100+0 records out
etc ... 

SQL> select GROUP_NUMBER, NAME, state from v$asm_diskgroup;
GROUP_NUMBER NAME STATE
------------ --------- ----------
2 PCASDGF MOUNTED
3 PCASDGR MOUNTED
2 rows selected.

The diskgroup is now removed. I then used DBCA to recreate the ASM diskgroup, all these disks showed up as Candidate disks.