A word of warning: just bringing out the tarrasque on these kids will probably result in one dead tarrasque, especially if you're not fully conversant with their skills. You're going to want to have a TPK "oops" that makes it look like you were just trying to give them a challenge after last game sesson. Here are a couple of ideas, though...
High level fighter or barbarian with maxed out physical stats (including books +5 in all of them) and a big, adamantium sword that radiates a continual anti-magic field when drawn. If you want to be really nasty and you have MotW, give him that anti-magic prestige class, say that he used the books before picking it up, and that he's cool with the sword since it is anti-magic. You should find most characters up a creek without their magical goodies, and a good, solid physical threat who takes that loss into account can put some serious hurt on them.
Similarly, make a dungeon full of illusions and Mordainkainen's Disjunctions, whittle the party down, give them several "false bosses" to waste firepower on. And take pointers from any meatgrinder you can find. We're talking nastiness on the scale of Spheres of Annihilation at the bottom of pit traps, concealed with illusions as a room behind the door, things like that. And when they get to the end, dose them with the biggest attack sequences you can find. In short, make it so that if someone does miraculously survive, you can still call it close enough to a total wipeout to want to start over.
Some people may whine about losing their kewl p0werz. Some may even threaten to leave. Truth be told, you kind of want to cut down your number of players, so unless you'll be left with just two people, don't worry about it.
And next session, start completely fresh, and focus on all the sticky little rules in the PHB. Point out that just because you thought you had it down last time, you may not have, and that this time you want to do it all right. It might slow the game for a while, but you want to learn your way gradually, and taking it slow from first level is the best way to do that.
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