Ok, just finished reading (or at least skimming) the whole forum, and I'm filled with rage and disappointment for humanity, and have a bunch of points to make.
First issue- full disclosure, I was that guy in high school, at least twice. Not quite *as* desperate, but close enough to have an idea what goes on in his head, and really, it's not that creepy. A *lot* of it is that underdogs want to justify their own self-worth, and the easiest way to do that is by feeling like they're the "good" guy. What you read as his creepy obsession, he probably reads as loyalty or being a romantic. Cause humans suck, nothing feels as "moral" to us as self-denial- pursuing something despite not expecting success, in this case.
You might ask why he doesn't consider the bother to you, but he doesn't understand that you're uncomfortable in this situation to- he thinks you could just flat-out tell him no, and he'd accept it. Sort of. More on that later.
Guys tend to be a lot more blunt about things- it's something I like about us, actually. When a girl says "I made it perfectly clear", she usually means something different than when I guy says it.
Also... this guy is clearly not socially adept. He probably doesn't like the position he's in, and doesn't feel a lot of hope- so he'll cling to any he has. If you leave any hole in your denial of him, hope will get through that like pressurized gas out of a not-quite-sealed container.
Rebuttals to a few bits of what I'd call truly awful advice.
Don't disappear for a while. He'll worry like hell that it's because of him, but he won't know, and as soon as you talk again he'll forget it, and maybe even think things have changed over the interim. He might be almost to the point of giving up and moving on out of self respect, then forget all that while you're gone.
Do *not* take any of the half-serious advice about claiming you aren't a sexual person. It'll feed *straight* into the whole good-guy thing. He'll be thinking "Most guys are obsessed with sex, and that's bad, right? So by still being attracted to her when there's no hope of that, I'm being good, and she'll see how genuine my affection is." This will be bullshit, but *he'll* believe it, or at least want to.
Don't be so hard on him. This guy isn't a sociopath, he's just confused- when guys get upset, we sort of break down- I think girls do, too. If we're jealous, we're more petty. If we're desperate, we're less aware, and we have less self respect. That's what's happening to him.
"He should take a hint, and if he doesn't, it's on him"- can't remember who said it, but yeah, BS. Sure, maybe it's his fault for not understanding what's going on; maybe if he were less self-centered, he wouldn't be in this confused place to begin with. But we should be aware of whether we're trying to find a good solution to a difficult situation, or whether we're trying to dole out Justice here- and unless you think his lack of maturity is such a crime on his part that it warrants the difficulty he's going through, just make it easier on him and close off all hope. Let him get on with his life.
So, that's that about that. As for the masturbation thing...
I got over the notion that things can be "wrong" for reasons other than having "wrong" consequences. No one is hurt by a guy fantasizing about a female friend- it doesn't make her life worse, it doesn't violate anything of hers, and, given guy's general lack of complexity or pretense about sex, it probably doesn't even change his view of the girl. Morality has no place in it- the only question is whether or not the guy is comfortable with such, and that doesn't really reflect on him as a person one way or another.