You have limited control here, which sucks. He can continue being a jerk, as long as he doesn't cross certain lines. Happily, you can continue not paying him, and he has no power to make you, short of taking you to court.
You might even suggest it if he doesn't give up on the money soon, just to get this whole thing over with. If he does sue you, make sure to ask the judge for court costs when you win.
In fact, given where you are located, would you consider going on one of those court shows? At least then it would be over, and you'd get a couple hundred bucks out of it.
In all seriousness, whether he sues you or not, you should write him a letter, and as drawmack says, attatch the email in which he states that your debt is clear.
Tell him, very directly, that you have paid your debt in full, and do not owe him another $150 dollars. State that even he has not documented you that you owe the money, exactly how much, exactly what it is for, or that that you agreed to pay. Accordingly, the debt does not exist, and he will receive no more money from you, and you will consider any further communications from him regarding this matter as intentional harrassment.
Keep statements like "I believe" and "I don't think" out of the letter. Just tell him how it is and how it's going to be.
Send it certified mail.
Gather all the communication about this that you already have, and from that point on, keep a good record of any time he contacts you (time, date, relevent quotes from the conversation, etc). Big things to note are if he ever threatens you in any physical or emotional way, or if he contacts you multiple times at work (these are the beginning of a harassment case, and even if they may not be enough to win such a claim, they are good to have on record.)
Again,
Good luck
