Getting someone to leave you alone without filing for a restraining order can be extremely difficult. If it is a stalker whom you are having a problem with, he will not stop stalking you just because you want him to, you must be firm and make it clear that you want nothing to do with him. Some people are persistent and the end result may have to be a restraining order, but there are some things to try first.
Confront the person in question publicly. Make him know in front of witnesses that you want nothing to do with him and that you will not be getting together, you do not want to be friends and you want him to leave you alone.
Get a second line installed in your home. Changing your phone number may work for a cell phone as these numbers are not put in the phone book, but anyone can get a new home number of someone who has changed it. Getting a second line is not usually detected if you keep the old one active.
Change the locks, or move if necessary. This should especially be done if the person you want to leave you alone was a roommate or live-in boyfriend or girlfriend.
Block the person from contacting you on your social networking sites. If you can't block her, then delete your accounts all together. You can always create a new account under a pseudonym, but any mutual friend you have will be a link to you.
Tell your employer of the problem you are having. Your boss will want to be informed of any situation that may cause a scene at your place of work. Let him know that you do not plan on letting this interrupt your work performance.
Contact the police. If you feel that you are being threatened, harassed, stalked or have any level of discomfort because of this person whom you want to leave you alone, contact the police immediately. Even if it turns out that nothing could be substantiated, the police would rather spend the time doing the paperwork on a potential stalker and have documentation than they would filling out paperwork on a kidnap or murder victim because they didn't take the threat seriously. This will also give you a record of the person bothering you if you need to get a restraining order.