That's so obnoxious. I've been in groups where that would happen. I've also been in the same groups where the group would decide to let a couple of people keep going OTF without responding and instead just keeping the ride going as per usual. The third scenario is that a couple of guys will throw in their own sprint point (that no one else knows about), and then the group gets shattered but like the more organized sprint points, there is an immediate regrouping.
I'd hope that this guy would realize that he's not only shattering the group but he himself is blowing up (when he goes to the back). I'd be tempted to give him a talk on group ride etiquette. But then if you've been with this group for a long time and are buddies with a lot of the faster people, maybe you could mention to some of them instead that they just let this guy go because it's ruining the ride for all but the strongest riders. This is the off season after all. If he's out there in no man's land without any challenges enough, maybe he'll get the picture. If not, well, you don't really want him in your group anyway so let him just hang out there 50 yards in front until he blows.