Add a condition to the action in which troops are purchased which checks to see if the player has enough points. If the points are stored as a variable, they can be compared using System > Compare variable. If they are stored on an object's variable, you will have to use Object > Compare variable.
Make sure it checks if their points are greater than OR equal to the cost!
Good luck - if you have any problems make sure to post your capx - it makes it a lot easier to see what you mean and how to fix it