How can I tell my Co-Worker to BACK OFF without causing work related drama? a little long, but 10pts!?

It sounds like you have tried to be polite, and it hasn't worked and you have tried to bring in your boss as a moderator. If you like the job, I would ask the boss if you could talk to them, one on one, and explain the entire situation. If you don't think that would work, maybe you should just find another job. Sometimes you can't fix the situation, and it is best to just leave it.