Hi there,
You could set up a timer which handles the amount of time a jump could happen.

In this way you can prevent that behaviour.
For example when you hit the space key, the timer starts to count, now you define how much time a jump may take. Just make sure that another jump only can happen when this timer has a certain amount.

You could handle timer-counting and reseting by traces, with a trace downward you always can find out if your ball/player is touching the ground or not.

hope this helps...

cheers