I used to have neighbors who did the same thing. They'd let their pit bulls roam free right after the animal control office closed each evening.
Do those dog whistles work at all?
Ex-Laxx mixed in with ground beef?
Puddles of strategically placed glue so the dogs step in it and won't want to come back?
Paint a mural of a giant wolf with enormous teeth?
Get a giant stuffed cougar to lunge at them, preferably one with a soundtrack of cougar growls.
Drink a lot of tea, then pee a perimeter along your property to keep them away. Bonus if you do this before sundown.
I'm sure you will immediately implement one of these excellent, brilliant ideas.
On a serious note, you could try sprinkling very hot pepper powder around the perimeter. I've heard they don't like it.