In the Settings --> Administration --> System Settings window, right on the General Tab. There is a section marked as "Set whether reassigned records are shared with the original owner". You can then choose whether or not to share reassigned records with the original owner.
What this does is any time you reassign a record it will share the record with the previous owner. Not just the original creator, but every owner after.
Note : This will not undo anything that is already shared, but it will stop the system from sharing anything more if you don't want it to.
