I recently spend a bunch of time trying to make a friend realize that he was essentially stalking some poor girl who worked at Wal-Mart. I gave him the following advice about persistance (which he proceeded to ignore, last time he got in her line she called for a replacement cashier as soon as she saw him, and he still didn't get the picture, he just thinks that persistance pays off).
Being persistant with a woman outside your social circle does not work, it does not work so much that it isn't even funny. That "He is a nice guy, but not for me" thought they might be thinking now will quickly change into much, much darker thoughts.