You're stuck with us forever! Muwhahahahaha!
Yes, I think Greg and Chroot are the only ones who can really delete an account, but there's not much reason to do so. We don't want someone's posts to disappear. If you don't post for a long time, your account becomes inactive automatically. I'm not really sure what exactly that does to your account, though. I guess that if you're leaving and not planning on coming back (how could you even consider it!...oh, wait, I considered it, and see how far it got me?), just go into your profile page and turn off all the places where you could get notifications or emails so you don't get bugged with things like messages that your PM inbox is full.