How do you tell someone you hate to step off?

Personally? Well, I tell them to back off and go away. If they don't then I cut them out of my social life and wherever possible avoid meeting them. Basically I send them to Conventry and ignore their existence. Thankfully I've only ever had to do this once or twice in my life, usually they get the message when they see me out doing something that they normally would have been invited to and weren't.