asm diskgroup drop from another asm instance

Question

I have created long time back one ASM RAC setup and I created one diskgroup with some name say xyz. I think that instance is gone but the diskgroup was not dropped exclusively using command

drop diskgroup xyz including contents;

Now I have create a new setup and want to re utilize these disks, i am getting below error

CREATE DISKGROUP XYZ EXTERNAL REDUNDANCY DISK ...

ORA-15003: diskgroup "xyz" already mounted in another lock name space

how can this diskgroup be removed? Any help is appreciated.