It's easier if you have mutual friends you can talk to, even if there's only one person you both know that can act as a mediator to send him the message.
If you really don't like this guy and don't want others involved, don't talk to him at all. He'll be persistent but you need to be persistent too. Even if the message seems important, don't respond. He'll give up when he realizes he's not getting through. It may take a while but he'll stop eventually.
If it still doesn't work, block his number. It's easy if you go online to your service provider and enter his number on the "blocked" list. If he's not forceful, he won't stalk you home and you have nothing to worry about.
If he sees you in person, be assertive of your standpoint that you carry no interest in talking to him if he's going to be bothersome.