Send the person(s) a letter, certified, through the Post Office (an address of the person will be needed). Request a Return Receipt and request Restricted Delivery if needed. If you are sending the notice to more than one person at the same address, then separate letters should be sent.
Be sure to make yourself a copy of the notice for your records and save the receipt from the Post Office. If the person(s) violates the notice then they can be charged with "Trespassing After Notice."
The Trespass notice can be picked up from the Sheriff's Office or download and print a generic form by clicking here.