It's really annoying when this happens. It's partly a Poser design problem - there should be a distinction between expression morphs and face shape morphs. What I do is this: after creating the character's face, with no expression, spawn a new morph target for the head. Then blank all the shape morph channels (in the case of V3, strip them out) and turn the new morph to 1.0. The character is now protected from all rogue pose files.