Quit letting them do that!
In all seriousness - you have to control your dip, and part of that is ensuring your knees track properly. I would suggest, without seeing you lift, that you should do a few things:
1. Before you start, make sure your knees are unlocked (not bent, just not locked in passive hyperextension like we normally stand) and your quads are tight.
2. Initiate the dip smoothly, which will mean a little more slowly than you're probably doing now. Let the weight of the bar push you down rather than pulling yourself down.
3. Push your knees out as you dip and as you start driving out of the bottom!
Some ideas for training the problem away are in this article - http://www.catalystathletics.com/art...articleID=1826