I saw this happening a lot on our club rides this past summer... The way I broke them of the habit on the rides I lead was to let them go up the road, while kept the rest of the pack together. As soon as we came to a convenient crossroads, the pack turned off the planned course and took an alternate route to wherever our destination or turnaround point was going to be. The "bunny rabbits" will get lonesome after awhile, and start looking for the pack to come up behind them... especially if it's an unfamiliar route to them and they start getting those niggling little feelings that they may have missed a turn somewhere.

When they finally caught up to us, they were pretty docile ;-)

Many groups, including mine, have a standing rule that if you run off and leave the group like that, you're no longer part of the ride, and you're on your own. It takes a fairly strong ride leader to hold things together so that everyone has a good time. Many clubs have gone to a "leaderless" ride format to avoid saddling folks with the adminstrative burden of being a ride leader, but this is one example where the ride leader idea pays off.