What is the best way to stop kids knocking your door and running off?

There is this group of kids round my way who have knowed our door 5 or 6 times in the past couple of nights. We don't answer the door but they persist in doing it. It isn't as if they have only started doing it and should get bored of it soon, they have been doing it on and off since last summer. Has anybody got any ideas on how to deter them?