Riot Control

[dmm]

The obvious answer is to abandon the planet, without scrapping any facilities. This gets rid of all population. (No, it doesn't make sense.) Now transfer all the pop in your transports back down to the planet. (You may need to wait one turn for the planet to forget that it was rioting; I'm not sure.) By the way, this is also a sure-fire, totally unrealistic, way to cure plague and to deal with wildfires.