Stand up. Right then and there. Tell them that it isn't OK. It can be as simple as "That's not cool. You owe me an apology." Is this from the general public, or is this from a peer group? I'm assuming it is a peer group. "What you permit, you promote." This was the topic of one of our recent training sessions (I work with large groups of children as part of my job.) If you don't stop improper behavior, you're promoting that behavior. Allowing it gives permission for that individual, and others, to continue in the future. I'm not saying it's your fault that people behave poorly. But if you do nothing to stop it, you're giving them permission to continue.