Friend made advances @ me i ended all contact. sends me stupid cards/mail. cops said ignore but i owe him $400?

Just send him a check with a brief note that simply states that you thank him for loaning you the money and that even though you both have parted ways, that you wish him well. If he bothers you after that, I'd go with the cease and desist letter. Better to end it w/out aggravating the situation further, if at all possible.