[Sorry, changed my mind.]

Quoted from other sources and tested: Population growth out of control? Give - as a gift - this settlement to your enemy, then claim it back with the original garrison nearby. You can enslave or exterminate the residence to bring them back to happiness.

[Enslave & population boom]

Huge-scaled enslavement of multiple settlements in a region actually makes the entire region very happy - through the population boom it brings.

For far-away lands where distance to capital kills, enslavment is certainly a better option than extermination. For every enslaved settlement, you will have slaves as a resource. Through trading slaves you have population growth bonus, from 0.5% to 6.5% (the highest I have seen so far, might go even higher). The pop. growth bonus effect seems to be accumulative - the more slave trade there is in that area, the more population growth bonus you get.

Then you get a nice public order boost of "population boom". That will help you stablize the settlement a lot... probably because everybody is busy at night, and don't get enough sleep to discuss revolting during the day.

If you have too much population, then just give the settlement away (the previous evil tip) and claim it back to enslave again. The effect on population is then same to extermination. evil evil.