[Eldar14]

I do believe the more important word of 'magic ring' is 'magic' not 'ring.' It is not the little band of metal attached to a finger that does anything, it's the magic placed in the ring wrapping itself around the sould of the wearer. I think it would be impossible to remove the rings from the ringwraiths, because they are a part of the ringwraiths. Even cutting off their hand would not work as it did for Sauron I believe. These rings were created for enslavement, in which case they would permanently bond to the person they capture.