You should send the person(s) a certified letter
through the US Postal Service. Request a return receipt and requested delivery if needed. If you are sending the notice to more than one person with the same address, a separate notice must be mailed to each person.
You must keep copies for your records and save the receipt from the Post Office. If the person violates the notice, then they can be charged with "Trespassing After Notice".
to download a Trespass Notice.