Uniquelynancy has the right idea. You need some proof that they did not clean the first machine. I was going to recommend that you take some dated pictures for evidence in case you do go to court over this. Document, document, document! The more paperwork the better when you have to convince the judge. Receipts, pictures, dates, notes--everything you can think of verify your case.
Do be careful about what you say because of slander/liable issues though. Be able to prove anything you claim. They sound like the kind who have dealt with "problem" customers before and may know how to use the system in their favor.