How can you legally forbid someone to enter your place of work?

It depends on where you work, who you work for, whether your boss(es) support you and why you're forbidding someone else.

If you work for yourself and own (or have exclusive lease) of your place of work, you can put up no trespassing signs and tell the person they are not welcome there. If they come onto your place after having been told before, they are trespassing and you can call the police to have them arrested and charged with trespassing. The drawback here is that no trespassing signs may scare away legitimate customers. If you work for someone else, THEY would have to put up the signs and someone in management would have to tell the person not to enter, but then the effect will be the same and the person could get arrested if they enter. Not all jurisdictions require "no trespassing" signs for the stay away warning to be legally enforceable, but many do and your boss may balk at putting them up.

If the person has been stalking or harassing you, you can ask a judge for an order of protection. You will need to explain to the Judge what the person has done (Frequent phone calls, visits) and/or why you are afraid of them (threats, gestures they have made, or prior violent/criminal history) You have to convince a Judge that you are being harassed or are afraid, but the first hearing to establish the initial, temporary order of protection, is sometimes done ex-parte, without giving the offensive person the chance to rebut what you say about them.