I find that a remedy is to go up to these loudmouths and either a) offer an opinion as to what they were talking about, adding that as they were talking so loudly I assume they wanted me to participate b) say that I usually don't eavesdrop, but in this case have no choice or c) say the conversation sounds really interesting, has he got a speakerphone so that I can hear the other person...

The person in question generally doesn't appreciate it, but they sure get the message!!!