I don't think it matters who started it. What matters is who believes it. You know it isn't true, this Mark dude knows it isn't true, so to me, that's the end of it. The problem is that you're letting it get to you. Try to not worry about it and ignore the people you think are behind it if you can. Once they see it's not bothering you, it's only a matter of time before it fades away into oblivion because it's not getting the reactions they wanted anymore.
Of course, the easiest solution would be to quit and walk out with your middle finger high in the air. Just make sure to tell the people you like that the finger's not for them before you leave