Guys all in white t-shirts!

great way for them to hide in the pack - if 1 commits a crime it is very hard to identify the individual.

a bit like picking out a sheep from a flock.

if they are thinking this way regarding the way they dress, I'd be very careful as they know how to play the game of not being caught if they are doing anything illegal.

I've played game with them also but very carefully.

Get yourself a sign made up that reads THIS AREA IS UNDER 24 HOUR VIDEO SURVEILLANCE and place it exactly where they hang out.

Then just nearby say 10 feet, get a friend to lay on the ground and outline your friends body with some white chalk. Put some tomato sauce where the head is and let it dry.
You will have to do this when they are not around.

As most crime is done by stupid people, they will think someone has been shot or killed and cameras are now installed in the area.

Then sit back and watch their reaction - it will have them out of there hopefully
However this could cause them to move on and give some else a source of drama.

An idea for the sign could be the JCPD logo and similar words regarding surveillance - cut and paste the logo from the internet and get some printable clear A4 stickers for your printer from the store. This will give it some creditability.