Oh this is just sick and wrong

[General Hawkwing]

Fellow 'would-be-rulers-of-the-universe',
you are missing a simple way to deal with rioting conquered planets. At least until a change like dmm mentioned "The unbelievable thing is that billions of rioting people would voluntarily board transport ships!!! The game should not allow that. You should not be able to move around populations until they get to a certain happiness level." is made.
Just include a few (10M-20M)volunteer 'recolonizers' on a small ship in your fleet. After taking a planet, if they riot then transfer the hostile population into a transport ship while dropping your happy recolonizers. Emergency build the urban pac. center and retransfer population if needed. This works best if the planet(or one of them) is domed, would hate to dome an undomed planet.