If you shoot them they will stop trespassing on your club's property.

Now to contribute something useful to the thread, If the signage doesn't stop these goons, making an example of one by having him arrested for trespassing (or more likely, escorted off the property by the police) will do wonders for controlling all the others. It takes one guy actually getting nailed for the club to get a reputation that they don't play around, and then other flyers will either join or stay away. If you budget allows, some security cameras near your gates or pit area to catch tag numbers would come in handy in the event of vandalism.