This is just a suggestion which I have heard before from good authority. If the money he owes you really doesn't mater then the best thing to do is to make sure there is no reason for him to call you if really have moved on in your life. Contact him one final time and say that you accept the money he has already paid you as full payment for everything (you say he pays you �50 a month so it is assumed you have received something), say that you really don't wish to have anymore contact. Ignore anymore texts and if you get a call then politely but firmly request that he doesn't call you anymore. Further to that you can get your number changed. Hopefuly the message will get through before you have to take things further. Good luck.