My father had a neighbor's bull that kept coming through the fence to breed his cows.(Santa Gertrudis bull on purebred Angus cows).

After a couple episodes. he drove the bull to the house and penned him in an escape-proof pen. The neighbor had to pay damages(reduced value of crossbred versus purebred calves) and also daily feed bill and upkeep on the bull while my dad had him.

Don't know if you have a way to round up and pen the cattle, but that would send a message that was clear to the owner. They could not legally come over and reclaim them without your OK.
You can corral and pen any livestock on your property.

I think you can post a notice in the paper and if nobody claims them, you can sell them.

I would never consider shooting one. Two wrongs don't make it right.

Better call a lawyer and take it to small claims court. You would have to prove damages and they would likely be based upon past crop production. Pix should show the damages.

Good luck.