How to disable the Guest account on OS X El Capitan
After some extensive testing (and multiple Time Machine restores--to verify that this fix works), I have found a solution. And it's pretty easy too, just don't do it on the go, or you'll wipe out your battery.


Visit Apple's dedicated website to download the OS X 10.11.4 Combo Update
Disable Find My Mac
Ensure that you have disabled the guest user

Install the combo update


Poof! Guest is gone.


Re-enable Find My Mac, if you'd like.
Disable guest user again, immediately after re-enabling Find My Mac


And that's it! It works nicely with FileVault 2, Bootcamp, and Time Machine, so no worries there.