I do it rather frequently.
- Own the zone's fortress. Links to the fortress provide stability within your forces. BTW: Minas Morgul will never be yours and "Iron Will" seems to be unusually common in new captains here, so just learn to embrace the chaos in this zone. FWIW: I tend to have 3-4 wild captains in Minas Morgul.
- Make your Captains feel like an important part of the team. Promote/Infiltrate as many as you can to Warchief. Make everyone else a bodyguard/spy to a Warchief. Fortress ownership means jobs for all. A pending Assault means up to 6 Assault Leader positions and up to 6 Warchief towers. Each Enemy Warchief tower can either support 1 Infiltrator or 3 spies. Infiltrators make your assault much easier.
- Support your Captains. Show up for their power struggles and protect them. Don't let them bleed out during Assaults/Sieges. For pit fights... well they're on their own there. (Homophone Hat-trick!)
- Punish betrayal. Use "Worse than Death" on those who betray you. A slavering reminder in your ranks of what happens to the disloyal seems to be a valuable tool. FWIW: you'll gain this ability when you finish the "Bruze" quests.
- If you do let a supporter "die", make sure he doesn't come back without your assistance. If you're in Act 4, raise any dead captains that still have their clan affiliation.
- Know that killing the blood-brother of a tame captain will definitely cause that captain to rebel. Just a trait to look out for.
- Don't die to a grunt when all your army slots are full of tame captains. He'll be promoted and duel one of your existing captains for the slot. And most importantly, It's just pathetic.
If you spend any length of time in a zone following these guidelines, before long, captains will stop rebelling, new captains will not have room to show up, and your own captains will stop participating in power struggles.
This stability can be a problem. As you gain experience, the captains in your stable zones will stagnate. So I also frequently kick the anthill.
Taking captains with you in an Online Fortress Assault, gives them the opportunity to kill enemy captains and gain levels. It also gives them the opportunity to die.
To re-introduce more general excitement/promotions into a zone, take advantage of the local pit-fight arena. At lower levels, sending one of your captains against new-blood can be rewarding, but before long, starting in Nurnen, even the "Champions" will be so unimpressive as to give your tame captain only a single level for defeating them. At that point go to the Army screen and send two of your captains to the pits to fight each other, and don't raise the looser. After 2 or 3 slots have opened up, advance time and you'll start seeing "wild" captains again.
Defending against Act 4 sieges can be a source of deaths, promotions and new recruits reducing a zone's stability.
Failing to defend against an Act 4 siege will do even more to reduce a zone's stability.
In any case, when trying to "level up" the orcs in a region, a confrontation between tame and wild captains may give a handful of levels to a victorious tame captain. Alternatively, taming the wild orc with a white gem in your "rune" slot will give him a handful of levels, and he'll feel obliged to quickly go on 3 or 4 relatively easy hunts/trials for another handful of levels each. Sometimes new blood is just easier.