You can sometimes remove them by heating the ring up gently with a small blow torch.This will make the metal ring expand and it is then easier to try and prise them off.They are pretty fragile,though,so care is needed!.
Incidently,there are new discs with the rings and wheel bearings already assembled which are quite expensive,but make the job of changing the discs so much easier.
Hope this helps,and good luck!.
Lagdti
September 2006