Kudos to you for trying to do something about the problem, and for initially approaching the issue with kindness towards the perhaps suspicious-looking young people with pitbulls.

I'm afraid I don't have any tips; however, I do remember hearing that pitbull fighting was a problem in some of the Boston neighborhoods, and that something was passed (or was in the act of being passed) wherein a police officer could demand that anyone who looked under 18 who was seen walking a pitbull had to show their license upon request. (I gather it was mainly a problem with young people.)

I can't find any information on the internet, but I do know that the person who told me this was involved with the Animal Rescue League of Boston (http://www.arlboston.org/Main/index.aspx)--perhaps they would be of more help.

Good luck!