The SAR requirements (criminal activity over $5k with subject, $25k without...) are the same for MRBs as they are for everyone else, so if you can't identify marijuana-related transactions attributed to the customer, you don't need to file. I would recommend reviewing the account carefully to be sure they didn't slip something by you and document the review if you haven't already done so. You could always file a voluntary marijuana termination SAR, but if they didn't do any transactions I don't think you'd be telling law enforcement or anyone else anything they didn't already know through the state registration.