If you know that the squatter(s) have food allergies, that can be a nifty little trick.
I like the lawnmower-and-hose idea as well, although you’d want to have a quiet-running engine...leave the hose in place beforehand and you can come by with said engine at night, hook up, and leave. Return when you’ve calculated the fuel will be exhausted, retrieve engine and hose.
If you live in an area with particularly poisonous spiders or snakes, this is also an option if you don’t have to put yourself at too much risk obtaining a specimen. (ie; don’t try to be the next Crocodile Dundee and grab a rattler barehanded)
If they’re squatting in a place that can be easily sealed off, like in a basement or storage hut, that’s an option as well. Wait a month or two and then come back, duly report what you find and tell the cops you secured the place until you had the time/money to renovate the property. There’s a certain storage yard in my city that is quite notorious for this tactic to the point that the locals call it the “Stay-Forever Inn”...try and squat in their nice heated storage bays and they’ll throw a lock on the door in the night and come back in four days. They ever catch you doing it again, they won’t come back for much longer.
And finally, if you happen to live somewhere near an abandoned mineshaft, let me just say that anything that goes down them is even more certain to stay there until Doomsday than a ship going down in the middle of the Atlantic. Especially if whatever goes down it - say, a large canvas bag - is followed by a couple cans of black powder with a long fuse and the resulting blast causes a cave-in.
Those are some good ideas. I’m not afraid of snakes but they’re not too prevalent around here. Old mine shafts are pretty common though. The problem with dead squatters is that bodies tend to trigger more vigorous investigation. My carbon monoxide treatment is probably a bit risky.
Food allergies is good. Along those lines are any neuroses or superstitions you can exploit that make your property absolutely unbearable. Squatters aren’t likely to be the most mentally stable sort and the cops aren’t going to investigate because ghosts or aliens scared them out. lol