Here is another tactic that a real estate attorney told me to use in a similar situation I am dealing with myself (this is not legal advice, you really need an attorney, run this idea by your attorney and see what they think):
Send them a certified letter that says: "Happy new year! This is a license to use my property for your crops and directly related work for another year! This license can be revoked by the property owner at any time for any reason, but I'll likely wait until after the harvest before I would do anything like that. I wish you luck with this years harvest!" Sign it your legal name as shown with the county, and "Property Owner". See what happens. If the crops aren't planted or the land worked, then leave out the part about waiting.
If they don't challenge the "license to use your land" in any legal way, than you might have a great defense in court if/when you send them a revocation of license, piss them off, and they take you to court trying for a judgement of adverse possession.
ETA: Maybe this won't work in your situation. Tell them that adverse possession must be decided in court, by those claiming it, put up your fences, give your neighbors your attorney's phone number, and stop talking to them. They just want to steal from you. If they damage your fences, then they have damaged your personal property, whether they think it's on their land now or not.