I bet homeless people started coming around, stripping down, and using the free alley shower on a regular basis. A little gross maybe, but better then smelly homeless people.
In a similar vein, I always thought it would be fun to install an array of wires with tiny spark gaps every foot or so on your building's wall. When your motion sensor detects somebody spraying graffiti on it, it fires off a neon sign transformer so every one of those gaps has a spark. That would ignite a solvent-based paint spray in a nice fireball.
I suppose to be nice, it should then turn on the showers to put out the human torch.
Of course, that brings up another question: Wouldn't it be easier to just string some electric fence wire up from ground level to three feet high along that alley wall? Nobody EVER pees on an electric fence twice!