Powerful characters can be challenging - but let their creativity and goals work for you rather than against you.
The Knight wants a griffon? Sure. But convince him he wants the BEST griffon - so now he has to go seek the best griffon. So he has to find this griffon - meanstwhile secret enemies keep trying to assassinate him by poison, sneak attacks, ambushes. No end of entertainment if you ask me.
Thief wants to join a great guild - sure. But they give him a nigh impossible thieving task as a requirement.
Elementalist wants to build a tower? Sure. But its got to be in the RIGHT place for best magical benefit. But there's a pesky horde of Goblins occupying the spot (and not a small bunch of wimpy ones either - I mean 5000 with spell casters, healers, 15th level goblin general heroes, etc). Its gonna take an army to move them off. So the Knight's gotta help. Hire the army, organize a supply line, actually make a military campaign. And that's not the GM's job - its the Knight's job to figure it out.
Which brings up a good point, your characters keep slaughtering your favorite monsters?
Use numbers and trickery.
When those PCs get arrogant, use numbers to see how they handle a sudden attack by a goblin patrol (60+) with goblin mages, clerics and a squad of Ogres as grenadiers. It can be exhausting and take 6 hours - but it can be loads of fun.
Plagues, peasant riots, insurgencies, extreme weather all make for great plot complications.
Use trickery, "Message from the King..." except its NOT! Watch them fall for it. Nothing more hilarious than silly con jobs to test your PCs acumen and leave them wanting. (BTW - works great for teaching children lessons about trust and verification)
But most of all. Open, plain trickery. Even veteran players will fall for the simplest of tricks.
Good luck!