Trolling your Facebook friends isn't so much a hobby as a necessity in some cases. Or at least for some people, but you have someone you'd like to get back at, there's a simple and very effective way of locking them out of their account, all you have to do is tell Facebook they died.Which you shouldn't, by the way, getting a profile "memorialized" sounds like a fun way to prank your friends, it probably is, but it's going to be a pain for them to get their profile reinstated.
So, unless you know they can definitely take a joke like this, there are plenty of other ways you can have fun at the expense of friends without doing so much damage. Just install one of those "funny" joke apps that are so popular.
Still, if you're curious about the process, BuzzFeed has a great
Facebook has a "memorial" option in case a user dies, which, at one billion active users, is quite often.
A family member or close friend can fill out a form and turn the profile into a memorial. All they need to do is provide some info and proof that the person is really dead.
The proof can be a simple online obituary. If your friend has a common name, it's not going to be hard to find one. Facebook doesn't do such a good job at verifying that your claim is valid, which is how you can trick the system.
Granted, with all the attention this is getting, Facebook is probably taking steps to ensure the process is safer so you should get to it as fast as possible if you're planning something like this (you shouldn't though).