I've been so fortunate to have coached with some outstanding coaches. Here is how one of them would approach this. He call the team together to talk about being a teammate and how good teammates benefit teams and how bad teammates tear teams down. Then, he say that he knows of a certain situation where "this was said." ... Then, he call out the name of the offender and say in front of everyone there that he knows that this player would not do this (even though he knew he did) and then say that he knows that because this player wants to be a good teammate and understands that bad teammates would be punished for these actions. Before very much of this coach addressing the issue, everyone would know that the coach is talking to that player in front of all of them. Then, he'd wrap things up by stating what good teammates do. This particular coach would then give the player some time to think about it and come up to talk about the matter. If that player didn't come up in x amount of time, the coach would then talk to the player in private.
I would see this coach do this and was amazed at how effective it was.