get to the front of the group, and "block". Literally blocking the road, takes several riders(even with the center line rule) and is generally considered inappropriate. But you can block by subtly disrupting the chase effort. You get to the front, and then slowly let the pace come down, forcing others to go around if they want to pick it up. Or when you get to the front you accelerate just a little bit, creating the accordian effect disrupting the smoothness of the chase group. Also, you jump on and chase down people trying to bridge, discouraging them from putting forth the effort because they know they're just towing up more team help to the breakaway.
Essentially you work very hard to get to the front, and then very subtly decrease the work. It's actually very hard because you have to work to keep getting to the front of the Peleton