If it is just for a dummy rounds, fill the case half full with some sort thick, but flowing epoxy (fibreglass resin or something). Seat the bullet, then lay the cartridge on its side for the resin to harden. That way, the resin will fill the case from base to bullet, and it will not be able to set back. It will, however, mess with the balance of the round, so they may eject funny. As a bonus, however, they will not roll around all that well.
For loaded ammo, you could try cannelureing the case. You use a machine to impress a ring into the outside of the case. This will (obviously) create a ridge on the inside, which will stop the bullet from setting back. Corbin makes a tool (http:// www .corbins.com/hct-1.htm) for about $140. You can see this explained more http:// www .dlsports.com/pistol-ammo-more-durable.html