I don't think anyone has a problem with players creatively skipping an encounter every once in awhile. It's just that most of the wizard skip options are uncreative (use a power from your sheet) and they can do it far too often.
Excuse me? Use in an uncreative manner? What the hell does that even mean? Using a power as it is written in the rulebook, and doesn't allow for any other uses?
You'll seriously have to explain how you expect someone to use a Flash-Burst bomb in a manner
other than a Flash-Burst bomb, and do something else other than blind a bunch of fools in a tight area for about a minute. [Colour Spray]
Or, just make them go to
Sleep.
I honestly see fighters and rogues used more in a more uncreative and more stupid manner than spellcasters are.
I'm guessing that you're fine with a wizard using their Silent Image to create an image of a fiendish monster; and have that scare away all of the encounters you wanted to throw at the party; because it's
creative (something you obviously get a hard-on for) and moreover, it's completely reasonable within the setting of the world (b/c let's be honest your enemies are going to say "holy crap, giant silent on-fire daemin knight,
ruuuuuunnnnn!").
Somehow I doubt that, and you're the sort of assclown MC that just likes to screw players; either because they've roflstomped your narrowly planned scenarios with a tiny bit of creative use of their powers; or because you can't create any sort of encounter that doesn't challenge the non-casters, but actively challenges the spellcasters.
Such as Uber-Runed hallways, with powerful spells written along the walls; but aren't glyphs or active traps; quite literally, they're spells; and a spellcaster can copy them down for use later. Remember, Traps are meant to slow, stop or delay entrance. Those things can be done by all sorts of methods; luring weak willed enemies with cheap or trivial bribes is good too. Also, making the group decide between abandoning new spells for the wizard to learn; or leaving the wizard behind to learn new spells (Mwah, Mwuahahahaha!) is a good idea.
Pro-tip: never split up the party. Was given to me by an MC who'd been doing it a bit over a decade longer than I had; it's very true, and an awesome way to make "higher level" dungeons better.
Force the PCs to make super sub-optimal choices; like splitting up the group; not using their magical equipment (local sensors will cast Dispell Magic every round that they detect spells being cast or magical items in active use, temporarily suppressing items and removing spell effects; most PCs will seriously not activate item if it means that they won't have to worry about it being turned off or on a cooldown later.
Afterall, a normal fighter can still seriously kill shit when they're level 10; they just can't kill on-par shit at level 10.
Some of my previous higher level games had the PCs handing away equipped items permanently in order to bribe creatures or officials or simply to keep Nightmare bandits at bay while in Hades. Since the treasure came at a very consistent manner, the Players were fine acting as if all of their possessions were transitory at best.
So... Frank's point on how items aren't "Owned", and are 'attuned' at best is a good basis for determing how to make PCs powerful or not. PCs should be encouraged to give away treasures, or use one-shot magic attacks; and be compensated over the course of the adventure, or by the next treasure payout the group finds (hint, hint, not that hard to do, you being the MC and all, seeding treasure is nothing to be ashamed of it happens in stories all the time).
so... unorthodox things to keep players in lower level mode:
1. Bribe the party such that they don't want to bring the high level spellcaster along; then give the Player a disposable set of creature
(s) for the briber to loan to the party for this next time-sensitive adventure
2. Throw hordes of weak-ass shit at the PCs; force them to not use more than mundane gear. Past level 6 this is reasonable; their natural numbers will allow them to plough through groups of enemies per encounter with no problem, even if they have no magical gear, actual Masterwork gear will be something they care about b/c they can Power Attack for just a bit more than usual.
3. Make the group split up the party any way that you can; multiple levers that need to be pulled at the same time; or other "triggers" for events and actions; or as in 1. out of adventure reasons for a PC to not be used in the adventure.