OK... it sounds to me like you want to keep this guy off your land and assure other farmers that he has no legal rights on your land. Someone else asked; What has your lawyer done so far?
I'll let others give you advice, as this is over my head; but some suggestions that others may want to comment on:
Is your property damaged by the poison, either temporarily or permanently? If not, you probably have no damage claim.
Do you have a copy of his alleged contract? What was your lawyer's take on it? What did your lawyer file with the court?
Have you notified him that you do not want him to trespass, and have you documented that notice? There may be a state law spelling out the type of notice to be given.
If he trespasses again, call the police and show them the documentation that he was notified not to trespass. If he shows a contract, point out to the police that your name as owner of the property does not appear on the contract (I'm sure others familiar with contract law can advise you of the legality of his contract with your predecessor in title, and how valid it might be).
If he is threatening other farmers with legal retaliation, there is probably some law against that. Document the threats, by affidavit, from those who were threatened. Also document how those threats have damaged you. (monetarily)