The best thing to do when trying to end a relationship is to be very straight forward. A person can tell when they can negotiate you out of something and when they cannot. The best approach would be to write the person and tell them exactly how you feel. Letters are good because there is no going back and you get to say everything you have to say without interruption. After you give them the letter make sure in closing you say your goodbyes and be clear you do not wish to have any other communication with them.