My own opinions on your problem..
It appears to me that most of the posters here are right. Your probably favoring a type of combatant that your Barbarian is easily able to slaughter, opponents with comparatively low AC or low HP. In general, for any given situations, same inputs will always result in the same outputs.
Now for wild guess work on my part.
Assumption 1: The problem is not with the enemy leaders, its with the enemy front line. The cannon fodder is getting mopped up too quickly by the barbarian to provide an adequate challenge.
Assumption 2: While using different opponents could work, the campaign situation calls for contnued use of this sort of cannon fodder. So if your adventure started with the players going after say, an ogre village, you cannot just switch to hill giants mid stream without breaking stuff. Also, if you, like myself, favor using swarms of lower HD creatures instead of higher HD creatures, upping the difficulty of the cannon fodder is a problem, and hard to do without slaughtering your players outright.
So here are my ideas for getting around your problem, given my incomplete knowledge of the situation.
1) Remove the Weapon. A well placed Sunder, or Shatter spell (if non magical weapon), would put your player in a bad situation. A Disarm from one opponent with another holding is action to pick up the weapon once its disarmed will put yoru barbarian in trouble if he has no adequate backup weapon. Casting command to 'Drop' could work also.
2) Create situational advantages. Set up ambushes where you can attack from the top of a cliff or castle wall. Any situation where he has to do something other then attack you or risk being killed will work. It works even better if your monsters can prevent him from getting close enough to attack.
3) Engage at Range. If the barbarian likes Melee, then dont engage in melee. Sun Tzu, in 'The Art of War' said only a fool attacks strenght with strength. If you cannot stand up toe to toe, then dont try. A two handed weapon is not so fearsome from a few hundred feet out. Even using Spring Attack or Ride By attack is better then staying toe to toe and eating a full attack from the party tank. Better yet, engaging at a distance with missile weapons with archers who attack / move away every turn will keep him busy. You can even extend this with the following tip.
4) Tactical Withdrawl. Also called running away. The best feat for any enemy you want to keep around is Run. When he rages, run the hell away, and return when he settles down and hit him again. If you put your opponents on mounts with mounted archery, you can keep away and keep hitting him. This will also get past those pesky buff spells.
5) Run and Re-enforce. When I am using humanoid opponents, I will make a point of having one opponent keep back, and use the Hide skill to keep out of sight. If his side loses, he runs for help. Keeping a horn handy works as a very good non-magical signal. Adding extra opponents mid fight will catch players off guard.
Things to Avoid if Possible:
1) Dont rely on a single high AC / high HP opponent to neutralize the Barbarian. A single target will get dumped on and destroyed very fast. Even if he has cannon Fodder support, he will be singled out, and the cannon fodder will likley just get mopped.
2) Dont have everyone dog pile the barbarian. If you have all the opponents dog pile the barbarian, your other party members will just nail him. Your not trying to kill the barbarian, you trying to keep him from wiping out your encounters.
3) Dont Nerf the Barbarian / Power Attack. Changing rules mid stream just to counter an overly successful player choice will only piss off your player. He probably knew better then you did that the power attack combo would work, and used his feats accordingly. Given a choice between adapting your players to fit your campaign, or adapting your campaign to fit your players, always adapt your campaign.
One last, very specific suggestion, assuming your party is about 7th level, and you are determined to dog pile your barbarian.
Set up an ambush encounter with a good number of Trolls. (Good number meaning enough to challenge the party). Get into melee before your magic users cook them. Make certain that a troll grapples the Barbarian. With regeneration, it can afford to eat the AoO. Once grappled, your barb cannot use his two handed weapon, and the troll grappling can still gnaw on him. And another troll can simply whale on him with his reduced AC. You will probably easily put the barbarian to 0 Hp, and the party mage cannot fireball the troll to prevent regeneration without cooking the Barbarian.
And if your really evil, do this near a cliff and have the grappling troll carry the barb to the cliff edge and leap so they both take falling damage. Trolls heal.
END COMMUNICATION
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