[Suicide Junkie]

The cheapest method is probably to drop police troops on your rioting planets.
Take small, stripped-down troops, and load 100-200 on a transport.

Then if some planet is about to riot, send in the shock troops and lock 'em down. With that many troops, they'll be in an enforced state of jubilancy by the next turn.

Then you can pick 'em up and move on. I reccommend having 20 police troops per planet, just for that extra buffer against rioters.
It only takes a turn or two to build them locally, and makes a great insurance policy. Even if things go really badly, and your planet doess riot, the troops will pull them back out on their own. No need to dispatch ships to all your rioting backwater colonies.