Use something that is obviously way out of their league, and see if they are dumb enough to try and fight it. Make it a roleplaying encounter with a dragon, or a death slaad, a balor or whatever. "Hey, you guys woke me up. Leave THAT sword and one of your band behind. A trinket and a lunch will compensate me for the disturbance."
The other thing that is really nasty: a pit filled with water, which is the lair for a gray ooze. Or any other traps, really, since there is no rogue.
Or a mob of irrelevant humanoids doing grapple, disarm, trip, etc. eventually they will overwhelm the cocky adventurers.
By saying that you don't really want to kill them, I can infer that the encounters you have been setting against them were weak. They will of course walk through any encounter that is specifically designed to NOT kill them, pretty much by definition. If you have encounters that repeatedly threaten life and limb, and some characters die, or you get a TPK, they will respect future encounters.
I agree, designing an encounter to simply wipe them out would be cheesy. But setting the tone that ANY combat is liable to be dangerous to them is not. BTW, find some way to get around life-restoring magics. Soul bind, or any of the souls-as-power techniques from BoVD (which destroy the souls) would do the trick. The knowledge that the yo-yo, alive-dead-alive-dead-alive-dead scheme is available cheapens any threat, even an overwhelming one.