Go to the part where The Dragons trash the restaurant. Carl sees where the gang spray painted it's name along the wall. That should be enough evidence to arrest them.
In theory, I'd agree that it was something of a signature. However, anybody could tag their name and pin it on the gang.
Furthermore, the graffiti does not prove the specific individual people who participated in the tagging or the trashing. It just alleges that at least one person in the gang tagged the wall, and at least one person in the gang trashed the restaurant. Maybe one person did both and the rest didn't know or didn't care. If so, who did it?