Get off my lawn!
Cyrozap suggested a Kinect could help a ‘bot someone stepping on the lawn. I don’t think it would be extraordinary to build something that would aim and operate a hose. Bonus points if I could get it to look like either (A) a scary sentry robot or (B) a big Santa Claus. Ideally, it would offer a warning, “Hey you kids! Get off my lawn!” And then a warning shot, just a wide spray – not intended to actually soak anyone. Then, if that doesn’t work – a directed spray.
And, then, well, there’s always the nuclear option. The ‘bot could turn on the sprinklers.