Ironbrokens are called that for a reason. I would STRONGLY recommend using my house rules on the matter (in my sig below), but now that you've already got one in your group, and being played by a powermonger as well, you're going to have to deal with it.
OK, FIRST RULE: One-trick pony characters do not deserve all the attention of a campaign or the GM. That means that you can't spend 99% of your time focusing on them. You can alter the crap out of your game to make it boring for them, but that usually makes it just as boring for the other players. It's best to make it MORE fun for the other players rather than try to make it boring for the powermonger. We all get these kinds of people and they will find a way to min-max their character no matter what it is, so focus on the GOOD of the other player characters. Look at their characters and give them a lot of background, NPC interactions, etc. Look at their career traits and give them extra benefits, etc.
Second rule: Next time, plan ahead with house-rules and limitations. If your campaign is going to be a social one, emphasize that to your players ahead of time. For example: I am currently running The Enemy Within. I had a couple rules for this campaign: Everybody must be human (human racial variant rules are ok), and I needed at least one noble, one bureaucrat, and one academic trait character. I told them that we would be using the social rules a lot more and that insanity would probably play a large role in things.
Third rule: in the future, outlaw the ironbroken "until you get a chance to re-evaluate it." Other broken characters include: min-maxed swordmasters, trollslayers with troll feller strike, mighty swing, reckless cleave, and thunderous blow.
Fourth rule: Breaking the Ironbroken: This only get's worse at they change to ironshield, etc. Here are some words I want you to mark in your mind: INSANITY, sewers, swamps, water (add 4-6 difficulty for all heavy armor), spells (heat metal anyone? create the spell), climbing (give 4-6 difficulty for heavy armor of all kinds), ice, coordingation checks, ladders, people with arrows. You don't have to even target the Ironbroken, you just need to make the other characters shine and show how WEAK his character really is when he is a one-trick battle pony.
Fifth rule: Have a replacement character rule. I would keep it simple: If players want to switch characters, they can do so without much/any penalty as long as the GM gives the OK. Have all players create a back-up character. This indicates that you aren't jerking them around when you tell them that PCs will die. Tell them that you like when players make a "well rounded" character. Expectations usually help prevent this kind of stuff.
6th rule: Dwarfs are resistant to disease, highly resistant to corruption, immune to mutation (for some reason), and the heavy armor just makes all of that worse..but they're not immune to insanity and they're not immune to social shame (afterall, one MUST withdraw once they lose), and they make terribly uncoordinated people (4-6 dice) when fighting on unstable surfaces (such as rubble-covered, wet, mushy, slippery, sandy, sloped or otherwise not-perfect ground). Whatever you do don't give him "fighting" ways to die. He'll just replace that character with another one-trick pony.
Anyways, best of luck