well there not many possibilites,
for new cats you just dissallow in ACP=>vbgallery => Modify Settings
section: User Category Options Allow Upload to NO
then the setting will disappear so user cant change it..
for existing ones it you can go to ACP=>vbgallery=>Edit Categories and change settings: User Options
Allow Other Users to Upload to Category => NO
BUT you have to do that manually for every user created category.
One problem will persist though, you will have to remove the images created by other users in a user's cat manually also.