If you have ever held a joint bank account with someone, and have removed that person from the account, go ahead and just close out the account and open a new one. Why? Because you probably had checks with both names on the account, and unless you have a super vigilant teller guarding the vault, your ex could clean you out without much trouble.
Your ex could also be passing off checks with various merchants. As long as the presenter has ID that matches the name on the check, why would anyone wonder?
Start fresh completely and save yourself the further heartache.