I just did it last night in order to get rid of unnecessary material groups (created in wings) that I used for establishing groups for rigging... It's a PITA but it's the only way I know of to get rid of the zones. Ummm, If you try overwriting with the grouping tool it just adds that additional zone.

Poser needs an in house way of deleting material groups. (Maybe there is a way that I'm not aware of).