It would be interesting to see if there is a way to rig up a pepper spray or lye or some other nasty source fluid instead of the garden hose, not that I would ever wish anyone any harm, of course.
There are CO2-powered "landmines" used in paintball combat games. Although they're designed to emit a spray of liquid paint, you could certainly load 'em with a mixture of Rit dye and the nasiest hot pepper sauce that you can find. Just remember to use extra caution when mowing the lawn. A small sign with a skull and crossbones that reads: "ACHTUNG!! MINEN!" might be a nice tough, too. ;-)
If anyone is considering using a paintball gun, remember that *frozen* paintballs really, really hurt. Just don't hit the vermin in the face, mm-kay?