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Okay here's what you need to do OP, do this right and they'll not only never know it was you, but they'll never come back.
Make a bunch of shellmines like pic related but load them with rocksalt, if they go over it it'll make a sound fucking noise and send rock up in their face, maybe fuck up their tires if you're lucky. Because the things are small and they'll probably bury it in mud when they go over it they'll just think their tire exploded, keep doing that and they'll leave you alone.
Big thing is make sure they're not loaded with lethal ammo because they'll get off their ATVs to investigate and could easy step on one.