I think it is in the hands of the supervisor although it sounds like they aren't responsive to the issue.

I do think you could be terminated on the grounds of insubordination based on your note. Understandably, you're frustrated but insulting someone (in documentation) will be ineffective and inopposite to your best interest. I do not mean this unkindly.

I've not worked in your industry so I don't know how much you're expected to clean up, etc. I imagine toilet paper must be installed. Your current issue may fall in a grey area. I'm saying you may be responsible for cleaning that "as is" whether it's the cause of your coworker or a patron.

Perhaps sit down and professionally discuss the issue with your supervisor. Don't confront your coworker or leave another derogatory note. That's not to your benefit. I wouldn't mention the note to your supervisor. If your boss mentions it, be honest about writing it. Otherwise, I'd not mention it. Keep the meeting brief.

If your employer isn't responsive, you may want to consider another line of work. Can you get out of this work task altogether? It sounds like you do other tasks at your job.

Good luck.