I would have politely explained that going forward you needed to be able to clean every time if you were to keep them on the books, and given a brief explanation of why and then put the ball in their court and asked if they wanted to continue. I've had issues like this before, done the above and customer has been as good as gold from then on, I've also had them cancel, either way you win, you either get a reformed custy or get rid of a problematic one, but the decision was theirs and doesn't lead to the potential bad social media reviews that might follow if they get the hump with you dumping them.