Long ago I was mixing with a group of people in quite a different context but we had a similar problem.

It was resolved with humor. If anyone got out of line someone would say loudly "Thow that Bum out of here"

Always got a laugh, made the offender feel part of the group because it was an "in" saying, but it got the point across.

If you start it off with each other, that is members of the old crowd, it becomes accepted easier.

Bert.