Werdnam:
I haven't dealt with
ki straps in my campaign specifically, but I do know that a lot of times, what's a problem in one campaign won't be in another, and vice-versa. I have no trouble believing that the straps have become a serious problem in your campaign, even if they haven't in others'.
First of all, I'd suggest trying to find ways to tone down the power of the items through tactics. This would allow the monk to keep the straps and get use out of them, without spoiling the game for everyone else.
Secondly, if you find that your campaign just can't deal with a monk who has +5 to the DC of his stunning blows, then take the player aside, explain this to him, and suggest a way to deal with it. Perhaps by giving the player a choice of a) keeping the gloves, but with a lower bonus (perhaps the enchantment was faulty and weakened after a while,) or b) getting another, different item (that you don't have any qualms with) in exchange for the straps going bye-bye. I find that balance issues such as these are best dealt with up-front with the players. That way, they know the situation, and feel they're being dealt with fairly.
Now back to the first-of-all. One easy way to deal with the fact that stunned creatures drop their weapons is to arm your foes with more than one weapon. Chances are, all your PCs carry multiple weapons. Daggers certainly, and possibly short swords, or even long and greatswords for fighters. Make sure your NPCs do the same.
I remember one time my rogue fought a gnome fighter who'd been tweaked as a killing-machine with the gnome hooked hammer. My rogue disarmed the gnome in the first round, and proceeded to punish him with his own weapon, because the gnome didn't have a backup.
Always have a backup. This holds for PCs and NPCs alike. Drawing a secondary weapon can be accomplished as part of a move, and doesn't provoke AoOs. With Quick Draw, you don't even have to move.
Since you mentioned that throwing out lots of lower-level critters would be problematic in your campaign, then this next suggestion might be impossible too, but in any case: try and use foes which have good saves. The DC with +5 is strong, but not impossible for foes with a good fort save. Try using more clerics and fighters, more fiends, more magical beasts, and, best of all...
more monks. There's nothing like an NPC monk to spoil the PC monk's party.
Of course, any and all of these tactics shouldn't be used all the time, since PCs
should be allowed to use their powers. And with that, my last bit of advice, stolen with respect from one of the masters of our hobby:
http://www.montecook.com/dmonly.html