Now this is something that interests me greatly!!
I saw it on you tube. A homemade electronic device shaped like a cross between a rifle and a bazooka. He demo'd it and it worked.
I'll try to find more this evening.
A full auto bb gun like the Drodz Blackbird would do it pretty quietly unless the guy was real close, then the sound of 30 second bursts might get him looking for easier targets. A 20 shot uzi worked wonders on local hooligans who targeted me. I only had to shoot it a few times at targets in the yard and they got the message.
This first one does the same thing but even if available , a ton of money because its new.