That situation obviously calls for some kind of Rube Goldberg device to drop down a bucket of water/paint/urine, or perhaps an anvil, when somebody enters the "no-no zone" and begins tagging the door, either with a spraycan or other tool.
Maybe just water sprayers that webcast viewers can trigger - not to spray the tagger, but to wash down the door making the troll's "masterpiece" run into the street.
I think water sprayers that people could trigger from the web would be a great idea. But only if they are left on during business hours.
As far as pissing in the corner goes, it might be neat to hook up a battery and the coil from a car ignition system to a piece of metal that might get pissed on.
But this is in America right? There'd need to be a sign that says "Warning, don't touch or urinate on this plate".
We have spent many many hours contemplating (fantasizing!) the endless variety of retribution systems for that doorway.
Wouldn't that just make people more likely to do it?
Probably. Which is a good thing.
Hmm, urine is saline. If there was a grate along the bottom of the door, it would run down there and could trigger an electrical switch to start the sprinkler...
Unfortunately, if you wanted to wash away the paint, you'd need to fill the sprinklers with turpentine. That'd stink worse than the urine, and sooner or later some wanker would piss on his door just so they could set it on fire.
My fence used to get tagged, but it was a white fence, and I had a can of white spray-paint.
"In better news, some filthy little gutterpunk both tagged, and then peed on, our front door this evening. Oh wait, that's not good news either! Well, if you happen to see the smirking little shitworm pictured below, please beat him to death with a lead pipe."
I have jwz on my friends list!