[mousepod]

Hey NA, the neighborhood where I work is rife with burrito trucks. What sets the good ones apart from the bad (I'm talking in neighborly quality, not food quality) is that the good ones have great big garbage cans that they use. And they clean up after themselves.
If they do come back, rather than hitting them with water, why don't you suggest they get a can and clean up after themselves? If you're feeling generous, you could grab one at the local Home Depot and sell it to them at cost. They already know that you're capable of calling the cops, so perhaps they'll take you seriously.