I usually try my best to be polite, ride along for a few miles, pass two or three at a time without getting crazy, waving to acknowledge fellow riders, etc., but ultimately you will have to blow past these slugs if you are going to enjoy your ride.
In researching passing etiquette on several group riding forums, it seems that many of these guys are offended when you don't follow thier group rules. While I avoid having too great a speed differential when passing, the size and slowness of these groups often requires going 130+ to safely get past them.
Respect, but get the f**k out of my way!