In Scotland we've had some success at reducing gang-related violent crime in areas like Glasgow, largely by going after the root causes of the issues. It was a mix of lobbying the communities these violent groups existed in to reduce support for them, trying to create alternatives for the actual gang members, and increasing sentences for carrying knives and other weapons used in violent offences. Glasgow's no longer the "murder capital of Europe" that it's been branded in the past, and violent crime is at a forty year low across Scotland.

Look into the Violence Reduction Unit if you're interested, there's been a bunch of interesting reports on it lately.