I usually give people like that the cold shoulder. If they have an ounce of intelligence in them they'll realize that they're not wanted around.

Though some people are thick and just don't get the subtle approach. I'm not sure an excuse will work on your friend to be honest. You may just have to flat out tell them they're annoying you. I've had to do that with my room mate, on more than one occasion.

But if I want to get out of something I usually just say I have work to do, or a big test to study for. Its temporary, but it works.

Long term excuse? Harder to pull off. Near impossible really. Its why I just use the cold shoulder method.