Depending on one's location, that's easy to say but perhaps hard to do. I fly with the same bunch of guys day in and day out. We do get pretty relaxed when it's just us at the field. There are people I implicitly trust, and I trust their judgment. Flying is a social thing as much as anything, and I doubt that the situation described here is very different. Sometimes people need to be made aware that they're making others uncomfortable.

Give them a chance to change their behavior before burning bridges. Being part of a club or a group is a social activity, and we shouldn't forget that. There are ways to deal with the situation that don't involve collecting evidence, making accusations, alerting authorities, and severing ties after making a mess. The goal is to clean things up, not to tear things down.