Yes, parents are generally ultimately responsible for their children's behaviour. If children are brought up with decent standards in the first place, there's a good chance they'll turn out to be decent people, and they won't be carrying guns and knives, or mixing with and admiring people who advocate violence. Discipline and education should start early - and the earlier the better. It's hopeless to wait until children are teenagers before parents actually say 'no'. It's true you can't watch teenagers all the time, but you can and should know where they are, you should know who their friends are, and you should watch younger children constantly regardless of their age. Parents who allow children to aimlessly roam the streets are reneging on their responsibilities. Say 'no' when 'no' is appropriate, despite of the tantrums, or in the case of older children, arguments, that will no doubt ensue. It's not the easy way, but it works, and in the end you save yourself, and your children, a lot of grief.
As for punishment, that's a hard one. I suppose the only way is to hit these parents where it hurts them most - and that's generally in the pocket - but make it tough - really tough.
By the way, anyone noticed the distinct lack of response here from the politically correct brigade who usually quite avidly find excuses the behaviour of hoodlums?