"Because of other DMs" ??
Do you have a single party in rotating DMs or something? If so, how frequently do you change off? (The reason I ask -- whatever changes get made by one DM/you could easily get undone by the next DM). -- of course, this could also explain "why" (because if they also have characters in the same group, upping the general power level of the group benefits their own character when they go back to PC mode -- but that's conjecture...)
As for the answer to your question (and assuming that you have some sort of rotating dm system or something), depowering a group and not pissing off people that are comfortable/like/implemented the power-up can be tricky. (And, if you are rotating DMs then anyone can undo whatever you do implement).
Possible suggestions (your results may vary depending on the group members)
* rob an item from one person (the one item that is most noticable as a power item gets stolen by a thief / wizard some how). Don't over do this and don't make it seem vindictive, just some item that has been shown to be powerful is obviously going to be coveted by others who seek power
* similar to the above, sunder items..
* undead that drain levels .... permanently..
* change the situations thrown at them - some antimagic area / dungeon that they are trapped in for several games. Creatures with high SR. Stuck on another plane where things operate differently. (basically, working with the upped-power to find adventures that could still offer a challenge)
* new group of PCs - or at least rotate groups for each DM so that each party is under influence of only one DM. You can pitch the idea as for continuity and world-ranging plots (if you think they may like that idea)! No need to scrap the current ones, just not for when you are the DM.
hmmm.. i'll stop there in case I am making an incorrect assumption about the rotating DM thing ...