a really gross question

This is why this forum is for innkeepers - past present and future, not really for guests. I have a gross question for you.
We have discussed vomiting and diarrhea before here.
If someone left a toilet full of the second on the list I mentioned above, not flushed, and messy, would you mark them as DO NOT RENT TO again, or not worry about it? Toilet is not stopped up, it is just not flushed and left in that state.

No, I would just think they were either rude or really sick. But not awful people. You know we had that problem quite a lot with the detached bath. For some reason, that toilet went unflushed a LOT and it was always what seemed to be a 'desperation' kind of mess . Always had to check it several times before we put an exterior lock on the door.
Perhaps afraid they would have clogged the toilet?
Remember my story about the guy who was really and truly horribly sick and we were cleaning the walls on the other side of the bathroom for days afterwards? The under side of the sink. The bathroom window. The bathroom door (about 3.5 feet from the toilet). This guy was major sick. I was just happy it was the detached bath because he had to close the door. If not, it would have been all over the carpet in the bedroom if the bathroom door had been left open.
He's been a great repeat guest for many years after that event.

It's gross, but I wouldn't ban him/her.

I do find it rude. And before Maddie's post, would think that a return visit would be too embarrassing for them to even consider.
I don't think I would put them on the DNB list unless a return visit warrants that it is routine.

I would assume they simply forgot. Flush it and forget it.

I would assume the person just forgot to flush, hard to believe as that may be. Would not DNB unless it happened again, which is unlikely.
I saw where the CIA is working with a major university to develop a pill that removes recent (short term) memories. You might contact them to see if they are accepting test subjects.

I'd flush it and forget it. Even if it was done intentionally, no permanent damage has been done. Except maybe to your mental status

my neighbour had an incident where it was everywhere up the walls in the shower like it has been smeared - I don't even know how its physically possible to do something like that.
On this kind of thing I would flush and forget

Yes, flush and forget about it.