Ya'know, I have never met a "fboy" who doesn't make their intentions clear. I think the issue is that girls simply fail to recognize or maybe even choose to ignore the signs. All I can really say is that you know that person is not good for you, so isn't that reason enough to back away? Exercise willpower, learn from the experience and invest your time into yourself rather than some idiot.
Also, for the record, I hate the use of the word "fboy"; if I were to use the word slut it would cause an uproar, however somehow the male equivalent is accepted.