You just charge for it. A dollar per entry should be enough for someone to come by and clean it twice a day, such that it doesn't matter what shenanigans antisocial people get up to in there. No mess will last longer than a few hours.

That appeared to be the norm in Italy when we visited earlier this summer. Most of the main towns in Puglia seemed to have at least one pay-per-use toilet.

Not sure why you're getting downvoted. This approach has been shown to work. The money isn't for funding the maintenance though, it's strictly to keep the riff-raff out.