If you have already asked the inmate to stop sending letters and they refuse, you should contact the prison facility. Speak to the prison administration or warden and advise them of the situation. Since all outgoing mail must be inspected and handled by prison staff, the letters from the inmate can be confiscated by the prison staff. Inmates are unable to go to the local post office to mail their own letters, so the prison staff can simply “intercept” those letters before they make it to the post office. If the method above is unsuccessful for some reason, you can always take legal action. You might have to get a judge to issue an injunction against the inmate that legally forces them to stop sending you letters. Any letters they send after the injunction is issued may be considered contempt of a court order, and that conduct may be punishable by fines or additional jail time.