Presumably, when making consistent money from a toilet, more resources can be allocated to keeping it clean and presentable. Possibly cleaned every few minutes. That would ba a horrible job.
I'd just lock the front door when they come in to use the restroom, inspect the toilet after they are done, and unlock it only after it's clean. Simple as that.
That would, in all likelihood, be illegal. In a lot of jurisdictions it'd be kidnapping.
I think the more likely charge, technically, is false imprisonment.
Who cares about legality, what a creepy awkward thing to do.