If you have migrated to another solution, I would think that deactivating BP and BBpress would eliminate notifications entirely. Email notifications from old mentions would seem to indicate that those forums are still active, or you have something else running that sends additional notifications beyond the initial one triggered when activity occurs.
@r-a-y created this plugin to toggle off Email Notifications settings if users had not explicitly gone in already and changed those settings. It might help you somewhat (it won’t cover the bbPress notifications though)
I miss-edited that. That should have read I migrated away from vBulletin to BBpress and Buddypress. I’m sorry! I will try this plugin though. Thanks.