I don't believe that violence is out of the question. Put up signs that say
<center>
No Tresspassing
Violators will be shot.
Survivors will be shot again.
</center>
and then act appropriately.
Lobby the police even more intensely. Use a camcorder and record the kids doing what they're doing, then use it against them. While you're doing it, tell them. Let them know that you're getting them on tape, to provide evidence of their crimes to the judge, and that if they don't want to spend their next couple birthdays in juvenile hall, they might want to think about ceasing their activities.
Look in to the laws regarding "citizen's arrest" in your area. In many jurisdictions, citizens are allowed to detain people caught committing a crime.
My favorite option for removing the problem is this: wait for one of them to breach your home. Then cap the fucker in the head, no questions asked. Nothing sends a message like "I will literally kill you if you keep it up. Look at your friend." Rationalize it by realizing that these kids are going to be even worse when they grow up.
Honestly, a camcorder and plenty of tape is probably your best defense. Video evidence can get things done.