I'll take your word for that bit, as I've never tried it.
But I bow to superior experience. :D
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The first thing that comes to mind is an Arduino-powered tripod-mounted mains-water-fed hosepipe with a webcam, actuator/server to aim & trigger the release of water jets. However, it would be subject to theft & abuse by local yoofs, and would also target children & postal workers unless you include a human-initiated sleep timer.
I actually have an unused custom controller from http://projectsentrygun.rudolphlabs.com if you want to pursue this...
I think i'm going to go down the route of making it hard for them to gain access by getting some of those spike strips, which will also help when the Zombie apocalypse comes into effect, then if if one eyed limping willy makes it through my defenses (i'm banking on him being the most trouble as he looks mean as f*** with all his scars etc) i'm gonna have some of that spray you can buy to ward the little ****bags off.....if they get past that than i think i have no options left but to buy 2 of those PIR noise cat deterrent gizmo's. :)
Practice dummy :p