Can't remove an erronuous file system

I have a file system I would like to get rid of.
It is faulty and the RAID it was created on is gone and the discs is now part of another RAID.
But I cannot add a filesystem to that one until this filesystem is deleted.
The filesystem is not mounted or anything, but nothing happens when I select "Delete".

What is the command to delete it from Putty, so I can get rid of it permanently?

The raid tab only have the new Raid I created and the fstab have no indication of this file system.
I tried to delete it before I created the new RAID too, but failed that time too.

If I delete my RAID on raid management tab, the file system "ru" is removed as well.
But as soon as I create a new raid with these discs, "ru" reappears.
I tried to unmount and deactivate with
umount -l /dev/md0
mdadm --stop --force /dev/md0

And then format the disks. And when I create a new raid the filesystem reappears again...