KarinsDad said:
Lots of good suggestions all around.
Another one I thought of is to send LOTS of minor damage area affect spells against the group (for example, a small cadre of low level sorcerers throwing 5D6 fireballs and lightning bolts from wands) over an extended period of time. Possibly along the lines of hit and run and/or ambush tactics for the spell casters. Not enough to even come close to killing the PCs outright, but enough to have them making a lot of saving throws.
Sooner or later, the PCs will roll ones. When that happens, they will have to start making saving throws for their items.
Granted this is an extremely slow process (and you will probably have to throw at least 6D6 spells if you want to destroy bigger items like armor or shields), but it has the advantage of the players probably not figuring out what you are trying to do (like various oozes might).
Sunder is another possibility, but do not overplay that card. One reason is that the enhancement bonus on the weapon attacking has to be as high as the one on the defenders weapon or shield, hence, if the NPC loses, the PC gets another weapon of that bonus (and your intent is not to give them even more magic). Dispel Magic in combination with Sunder against a given weapon can be potent (you could destroy a +5 sword with a +0 sword that way), but if you do that, then your PCs will know that you are attempting to destroy at least one item.
Admantium weapons gain a nifty Sunder bonus as well. A few Duergar (Invisible as well) attacking said Dwarven barbarian for the obvious reasons spelled out in their background (he being the wimpy-non-evil-variety of Dwarf) give you easy justification for them targeting him.
I used to play in a game (Rolemaster using the the Underdark as a converted setting) that used the Ars Magica style of revolving/sharing DM duties. I was the group's DM for everything, and, sick of never getting to play, I told them all that we would run a game where we all took on the duty(each DM would have theor character tag along as an NPC). Most of them jumped at the idea, since it allowed for short games/ideas and the more timid of them didn't have to plan out long campaigns...
Well, I was 4th in the rotation, and by the time the DMing duties got to me, we had had the 'Oops, that NPC item is worth a fortune, oh well...you guys have 10,000GP each...' didn't-think-it-through-guy (1stDM), the 'Magic Items?, Sure, you all have 10,000GP (at Level 2, no less) each', check-out-these-wicked-items-Monty-Haul-guy (2nd DM), and the 'You aren't all that tough, guys...' try-to-murder-the-party-guy who threw hordes (over a hundred) of Wights at us (at 2nd-3rd level no less), we moved faster, thank Lolth...
Anyways, when I took over the reins, deciding that I would just (sigh) be the proper DM for the game (which had some very cool characters, truth be told), I was stuck with a bunch of monty characters and some poor ones (that had joined the game late). I delt with the problem in game. These characters had higher-ups, Noble Drow in this case, who didn't exactly like flashy Underlings running about showing off their +20 swords (+4 in D&D). Solution: Some items were confiscated in-game for the obvious reasons (the greed of the higher-ups), some were left. We had an advantage in Rolemaster in that (with such a brutal critical hit system), a lot of the money was spent on healing herbs, which only the most horrible and callous DM would take away due to their value, but there were enough rediculous magic items that I had to do a bit of clean up.
We face a similar (possible) situation in Living ENWorld, where I am one of 3 Judges. With several folks running games, there is bound to be th situation arise where DM5 has to deal with the stuff that DM4 gave out on his/her adventure. At least we have Judges to oversee and attempt to regulate power levels (Dms have to have anything big approved by us, lest the balance shift in a wacky manner).
My suggestion regardling the amount of magic items follows as above, sooner or later, those fireballs etc..will get rid of some of the volume.
I'm not sure how many spells your Wizard has access to, but I am not of the opinion that even 3 or 4 times the amount of
possible spells for them to pick from is a big deal.
So he has access to 30 1st level spells, big deal. He's still only memorizing 5 a day (or whatever). It's the barbaria (or whatever) with the massive weapon that is the problem (as you have stated).