This kind of stalker believes that he is in love with you. To show his keen interest, he keeps calling you, dropping by, writing e-mails, doing unsolicited errands "on your behalf", talking to your friends, co-workers, and family, and, in general, making himself available at all times. The erotomaniac feels free to make for you legal, financial, and emotional decisions and to commit you without your express consent or even knowledge.
Moreover, no amount of denials, chastising, threats, and even outright hostile actions will convince the erotomaniac that you are not in love with him. He knows better and will make you see the light as well. You are simply unaware of what is good for you, divorced as you are from your emotions. The erotomaniac determinedly sees it as his or her task to bring life and happiness into your dreary existence.
The best coping strategy is to ignore the erotomaniac. Do not communicate with him or even acknowledge his existence. The erotomaniac clutches at straws and often suffers from ideas of reference. He tends to blow out of proportion every comment or gesture of his "loved one".
Boss: Next time he calls just hang up, or better yet; pay attention to the caller ID and don't pick up at all!
Secretary: Didn't you break up with him like 3 months ago?
Boss: I did. But according to his psychologist, my ex suffers from Erotomania. That's why I've broken up with him over 20 times, but he just can't take "no" for an answer, despite the fact that I got a restraining order after he got his hands on a fake marriage certificate with our names on it and showed up with it at my parents' house during the Christmas holidays. The psychologist tells me that ignoring an erotomaniac is the best thing to do to make them back off. So now, I'm ignoring him completely. I'm pretending that he doesn't exists and I would appreciate if you did the same.
Secretary: Of course.