physics - How to restrict jumping to a single jump?

I'm making a VolleyBall game in HTML5 with PandaJs GameEngine and plugin is used physics and Box2D.

The problem is that players can jump, but they can jump over the net with multiple jumps. I have already implemented collision detection but i don't know how the restrict move space for player. I also attaching my screen shot here so look at this :

In the picture Player1 goes Player2 side and vise-versa.