Most of the books I read don't involve masturbating in windows though, so perhaps you should offer me a suggestion?
The original post said he's getting "increasingly public with his masturbation". I don't know what that means, exactly. Does it mean he's sitting at a desk that happens to be next to a window? Could it be that he left his blinds open when he was getting his rocks off? Maybe. Hell, I keep my desk near a window too. If there's a crack in the curtain when I'm taking care of business, should I go to prison?
My point was there is a very thin line between being an exhibitionist at your window, and spying on someone in the privacy of their own home. And it would be extremely hard (if not impossible) to prove that HE was the one committing the felony, rather than yourself. Who makes that call?
And regarding the "standing in a towel, flailing his dick"? Who among us hasn't done the helicopter after getting out of the shower? Who among us hasn't realized forgotten to close the blinds until we dropped the towel? Again, it's a case of whether or not the guy intended to showcase his goods to the world, or whether you're just peeping into his apartment getting a show. It's hard to prove motivation, and could easily wind up a case of he-said, she-said.
It's also in pretty poor taste to mention the rapes/gropings. After all, if he's at his computer 24/7 (or anything close), he probably isn't raping people too often. Accusing people of crimes without any kind of evidence is called slander. Writing it down is called libel. Those are also against the law.
The moral? Just fucking deal with your neighbor like a grownup. Go over there, knock on his door and say "Hey, dude.. shut your blinds. The whole neighborhood can see your junk." Maybe he is a pervert, maybe he's just some awkward guy who doesn't realize glass works both ways. But you can't just assume the worst, and try to send him to straight prison like some of the "sheriffs" in this thread are wanting to do. Personally, I'm a believer in the whole "innocent until proven guilty". And so long as there is a reasonable alternative, you can't just automatically assume someone is the worst of the worst. A guy wanking it at his computer does not mean he's a raping, groping exhibitionist pervert. Chances are, he's just some guy that doesn't have a girlfriend.