Is it too late for me to jump in here and add my comments? This is just my interpretation of the situation, so please feel free to disregard. I'm no more expert than the next person.
But on the most basic level it sounds to me like what this guy is doing is trying to establish dominance. If he's hitting on you when you already have a boyfriend, he's trying to assert himself over both your boyfriend and you. The best way I can think of to handle it is to put the kibosh to it, as quickly and completely as you can. I think Pierrot is right: it's probably going to take several encounters before you get the idea across. Bearing that in mind, it might be a good idea to tell as many of your friends and family as you trust with the info.
I don't recommend writing a note. It might help you to get your point across, but it is also going to look like you're afraid of confrontation, and he may see that as a sign of weakness. Also, he can always trash it and pretend he never saw it. What I think you really ought to do is meet him on
your ground--close to your flat, for instance--and talk to him face to face. Bring a friend, if you need to. Do you have any casual guy friends who might be willing to help you out, or maybe one of your boyfriend's crew who'd do you the favor? I'm not saying a man would necessarily be able to handle the situation any better, but if your neighbor is half the pig he sounds like, he might not take a female friend seriously. One friend only, though; any more than that will look like a threat.
Which brings me to this other point: the other thing to remember about a face to face interaction is
not to come across as too aggressive. If you're rude or antagonistic, you might set him off. Keep in mind that, in his own way, he's probably as emotionally worked up and invested in this as you, it's just that his emotions are different. Again, Pierrot is right--just be blunt. Don't be overly formal or pleasant, but don't be rude. Just tell it like it is: you have a boyfriend, you're not interested in another one, and the way he's been coming onto you is offensive and off-putting. Don't offer to be his friend; don't negotiate; don't give him a consolation prize of any kind. Say your piece, and be done.
I know I've written a whole paranoia academy lecture here, and I'm really sorry if you're sitting in your chair going
But taking control of things before they develop any further is important. It will help you feel a lot better--more powerful--and it will help defuse this, which deserves to be taken seriously. Take care.
Lon