Hail!
The short answer: you can't.
People bitch and complain. It is what we do. This is reflected in the game.
There are things you can do- especially as a Roman- to slow unhappiness down.
I always (ALWAYS ALWAYS) put up taxes to Very High, and move my capital to the economic center of my realm. I also garrison almost exclusively with Peasants. When unhappiness creeps in, I bump up the garrison by a peasant or two. If that does not do the trick, I lower taxes and being building an army to take the city back (I usually keep a theatre army of some legionaries, some archers, and some onagers within each group of cities for such an emergency). When the inevitable revolt comes, I rush it, exterminate for the instant happiness, then use that city to retrain my forces. Next round, set up taxes to very High and start the cycle over again.
Note- you do not need green faces. Blue and yellow are fine, with Blue being about (+/-) a turn away from red. Red is bad- do something or suffer the revolt. I am bloody- I take the revolt, crush it utterly, rape and slaughter the ungrateful pixels that dared defy me, and let the survivors refill my ranks with hardened warriors. But that is just me.
Romans can also shorten the interval of Games and races if they have a circus maximumus and arena. These also make the peoons happy and forget their troubles.
If you have a huge city with a massive population and there is a plague somewhere, send a spy to get it and bring it to that city. It will burn out after a while, but will lower the population.
I also set Peasant Production to 0 turns, and since I play exclusively on HUGE, I can pump out 2000 peasants per turn. TYhese I send to where population is needed (or to enemy provinces) and there disband them into the local population. I have caused other faction's cities to revolt in this manner, most recently I took Nicomedia from the Greeks via the Rebels through this.

You can also modify the game to have walls give a law bonus.[This message has been edited by Terikel Grayhair (edited 04-03-2014 @ 02:56 AM).]