Force moving the MOUNT will cause the rider to dismount, knocking the mount prone will also automatically dismount the rider. Knocking the RIDER prone gives the rider a straight saving throw to remain mounted. Basically if you want to dismount someone then attack the mount with some power that either causes forced movement or some condition that results in the mount going prone. Beyond that you can often make the rider WANT to dismount, immobilizing the mount, grabbing it, slowing it, or stunning it will often force the rider to choose between dismounting and simply wasting his turn unless he's got effective ranged powers.