| You've been given a lot of replies here, but one that has been missed, is to GIVE THEM what they think they want!
Stop, tell the PCs you have an idea for a SUPER one-shot adventure, and you want to run it in x weeks.
Have them create characters of some high level (10th if that's high for your group, or even 15th or higher). Let them equip the characters to the MAX according to player wealth rules, and then GIVE each of them some "super item" of your own design.
Then run them through a nice, easy lower-level dungeon that presents no challenge to them. Let them smash everything. Give them tons of loot. Every time there's a rules question, just say "of course it works your way" and make a favorable, easy, walk-all-over-it ruling.
When they ask what's going on, tell them you're certain it is just what they've been asking for for the past few months. Be serious. You're not kidding, or being dishonest. You're meeting their every demand. Give exactly the same treatment to ALL the players.
I bet they'll have a blast for about 3 hours, and then get bored. At the end, ask them if they would like to DM for a while, or if they'd like to keep playing this game ad infinitem, or go back to the regular campaign where there is at least a modicum of control and challenge to the game....
(And then doublecheck to be sure you're not being a skinflint; and raise the difficulty/death level a tiny bit - if they're slightly more in fear of death, they may be less inclined to gripe about treasure and such). | 
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