Player is set on doing something MASASIVELY stupid, needs to feel consequences

[Tawmis]

Long story short, A level 6(2Cleric/4 Sorc) Star elf (Sea elf stats) Just got back from the Underdark with his party so he KNOWS how dangerous it is down there figured he didn't get to meet any Drow. So now during down time he wants to go back there BY HIM SELF. To find and say hello to the Drow. Which I made very clear In character and out that the Drow are NOT friendly in any way shape or form.
So outside of overwhelming him with Drow spell casters and soldiers to for ever enslave his character, what else drow like stuff can I use to confirm a just punishment
PS, he believes he has a way to escape...As the dungeon master I highly doubt a level six multi classier can escape from a Drow city, in anyway shape or form. Again I want a punishment that'll stick
Update: He didn't do it. I throw some stuff at him that was more interesting to his character, and I was able to search my cities and find a surface Drow for him to meet...Now he just wants to say hello to a Drider next time he gets a chance. Oooooh. Yeah.

Next time, let him do it. Let him be taken prisoner. You did, after all, warn him.
Have a random character, a level below everyone else, in the ready - and slide it over to him and say, "There's the character your playing until the party rescues you."