Who knew? Locking out a friend's account on Facebook is actually very easy. But before we go any further, a word of warning: This is pretty messed up, and we can't in good faith endorse doing it. (Unless, I guess, it's exam time and you're looking to help a friend become more productive.)
Alright, if you're looking to screw with a friend/enemy, what you need to do is designate your buddy's Facebook page as a "Memorial State," which normal, non-terrible people can use to keep a Facebook page alive for someone who has recently died. What it does is transfer the person's page from an active account into a memorial page—a virtual gravestone, if you will. This also locks the person out of his own account, because, you know, he's supposed to be dead.
Buzzfeed first pointed out the prank ramifications of this (and for an example of how they actually went through with it, head there), but here's the jist: You go to this page on Facebook, and you fill out your friend's name, email address, and Facebook URL. Then, you give your relation to the "deceased person," and you link to an obituary that will provide proof of death. The obit should just be of a person who shares a name with the person you're pranking. Buzzfeed submitted one of a recently passed 74-year-old who shared no qualities other than the same name as the 20-something whose account they were trying to lock out. This actually works.
The person will then be locked out of his account until he fills out this form, which forces Facebook to reactivate the account.
But, odds are, he's not going to know what the hell is going on for a while, much less know where to find the help form. So this evil, evil prank could last for a long time.
(Remember when a Facebook prank was just writing an embarrassing status on an open computer? Yeesh, time have changed.)