How to tell a guy who is ANNOYING, that you are NOT interested!!?

In homeroom there is this guy who is ugly, dumb (like who doesnt know anything about what were studying), has a face full of acne, you know the ugly kind of guy no girl wants. so yeah, how do you tell this type of guy you are not interested and NEVER WILL BE! i mean he keeps on staring at me and i feel uncomfortable. help anybody? :(