Yell at Them
Tell them to go to a place where there are no lemonade stands.
Laugh at Them
A good technique: start with "ha" and end with "hee."
Talk About Them
Preferably to a friend, over a cup of strong coffee.
Scare Them AwayAny mask will do, but any beast or vampire getup is particularly effective
Dress Them Up
With the right attire and hairdo, you can make anything seem attractive and desirable (to someone else), right?
If they are under your control, you don't have to worry about them.
Ask them the hard questions, including: where, what, why, when, and how? Then get a mug shot.
Boss them around like the scary dude in the fifth grade who made you cry. You're bigger than they are --don't let them forget it.
Let them go. Tell them to have fun, but not to come back.