What you need to do is if he touches you creepily, grab him by the wrist, pull it behind his back in such a way that it hurts. Other arm around his throat, and say in the most calm voice you can "Leave me and my friends alone. If I catch you following anyone creepily, I will not hesitate to hurt you." When you release him, push him away and say "Scram."
When you're being questioned about it later, say it was justified self defense.
But in all seriousness
If you notice him continuing his stalking thing, reporting him to the police is always an option.
