If you use dkp, give dkp penalties.
Sit as many of them out as possible everytime they disagree to sunder.
Giving them a link to this thread might assist in convincing them.
They are childish and selfish if they completely ignore even a raid leader's order to sunder instead of simply forgetting about it which would be somewhat understandable if they never had to do it before. This kind of attitude is extremely unhealthy and I don't think it's far-fetched to guess they're one of the special snowflakes that rather than following proper tactics make their own up to benefit themselves (dps-wise, f.ex).
Bad behavior does not only affect the amount of dps done by the raid group, but also the other players mentally. If things simply do not get better, I would seriously consider kicking at the very least the worst one of them.
No matter how casual a guild is, raiding is a team game.