There is someone who stares at me a lot. They know I don't like them the same way. I don't like him at all, in fact. What is the best way to tell him to back off?

If you can, be nice about it to him and just tell him : Look! I'm not interested in you so please don't bother me. Might as well be straight forward but polite with it. Any boy in his right mind will back off then. You watch and see. Sometimes you have to be firm, but polite and show manners. It'll work.