Preferences>VST settings>VST3 Migration
couple of check boxes there.
Also you can exclude plugins in Plugin Manager.
Or, Rob is right move the dll's of VST3 plugs you don't want to use.
Of course, excluding or moving would make them unavailable to new projects.
BitFlipper once said;
"Who needs VST3 anyway?" I think I have that right.
ARA requires that Melodyne be VST3?
T