I agree - has the boss said "Firmly tell this person to leave, and never come back," "Firmly grasp the telephone and dial 911, to summon the police at once," or "Firmly tell him that this is not appropriate public behaviour, and you're requesting (firmly) that he stop"? Because the first two are appropriate measures. The third is useless. What we commonly think of as "polite" involves "not demanding that people do as we want". This is not the appropriate choice when someone is breaking the law and/or acting in a way that is a million miles from accepted public behaviour.
If you want your library to survive, let alone thrive, you must not allow this sort of behaviour to continue. Therefore, asking, "Please, don't do this here," is not an effective response. "Hello, police, I wish to report a crime in progress," is. "Get out of here and never darken our doors again!" would also be appropriate, but only if you have the resources to back it up.
Let's say he was stealing your handbag - you are not required by etiquette to ask him "Please, put that down at once." You are allowed to scream "STOP THIEF!"