There is no way to know if a person receives an email, so you cannot assert that the person received any legal notice of his right to enter onto your land.
Thus, you need to either send certified mail, return receipt requested, so that you have a signature to prove that notice was given, or you need to have a process server hand deliver the notice to the former employee.
Alternatively, the supervisor can call the police and request a criminal stalking restraining order be issued against the employee. Or, the supervisor can go to civil court and request a civil harassment restraining order.
Finally, as employer, you can instruct the supervisor that the next time he is harassed by the employee, that he can dial 911 and ask for assistance (or, he can call the regular police line, and ask, if it doesn't seem like an emergency -- but for the police to arrest for criminal trespass, the officer must generally observe the suspect actually trespassing.
Hope this helps.
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