Trying to top their power inherently means handing them more Exp, for the higher challenge ratings.
That in turn means that they advance in levels and power faster, and pick up bigger treasure items.
Stick to lower level creatures that present a challenge as a group, but that, by the numbers, are small change in the Exp department. Spice these with a few Big Bads.
Also, instead of trying to give your monsters high ACs, which the non-power gamers will have a problem with, look at Displacement and Concealment effects that give a miss percentage. AC10, with a 50% miss chance is the same for everyone.
And you don't need magic to do that. Smoke works. So does thick underbrush.
In my group we have one and a half power gamers. I know that sounds strange, but one player continues in the near-mindless pursuit of "more power".
We do treasure splits by equal shares based on resale value, then you "buy" what you want from the pile. Anything not bought by a party member gets sold on the open market, and the proceeds then go to the players based on any unspent share they might have.
Our power gamer owes people, big time, for past treasure picks, when what he wanted was more than his share.
Our latest pile includes a Ring of Freedom of Movement that he is drooling over. Also several +6 stat items. And, naturally, he wants them all.
He's pretty much said that he's going to have to go deeper into debt to get what he wants, but he's willing to do that.
My character is the one who's been loaning him money, since there aren't that many items that a Bard can really use. He's just assuming that I have the money, and that I'm willing to front him more loans. Note that he hasn't made any effort to repay the ones he has, even when he had cash.
And he'll be mad at me for cutting off his credit, just you wait and see.
Our "half a power gamer" came into the game having played at tables where the power game was required due to predatory DMs and PVP from others at the table. He's coming to realize, slowly, that this doesn't fit our style, that nobody's going to stab him in the back and the DM isn't going to set up an ambush aimed just at him.
He's coming around.
I have no hope of "curing" our power gamer. He honestly sees the acquisition of power as the goal of the game, and he intends to "win" When he gets slapped down, as we demonstrate obvious flaws in his "perfect" build, or show him where he misread the rules (probably on purpose), he just becomes more determined. In his mind the slap down wasn't a response to his power gaming, it was proff that he wasn't power gaming enough. After all, if he'd done a "better" job of it, that slap-down wouldn't have worked, would it?