Then read this Instructable and you'll find out.
Warning: Not 100% guaranteed
Step 1: Tell them
If you have enough guts to, and are not afraid of akward moments, then you should just tell them they are annoying. This step is also dangerous in that it may get you hit.
Step 2: Be annoying back
If they are annoying you be annoying back. This is simple, just constantly talk about random things and poke them a lot. This will usually annoy them and they will leave (maybe). This works because most of the time they want a funny reaction from you but if you be annoying back then they will see that it is futile to continue annoying you.
Step 3: Hit them.
Just hit them up side the head. Sure they will probably get mad but at least you feel better.