Best way to end a friendship

How do say your friendship is over.

My fiancé doesn't know how to tell his friend that it's done. His friend has been awful to us and thinks he's done nothing wrong. My fiancé has tried over and over again to give him opportunities and nothing has worked to fix the problem. My fiancé has had enough. He wants to put it to bed. End the friendship and move on. What would you say?

Edit: he already tried ignoring him. He can't keep away.