Ok there is something really broken in the User options in OS X. You can disable the guest account even though FileVault and Find My Mac are on. These are the steps that worked for me:
Step 1. Turn off the Guest User (pay attention, buggy dialog!)
- Go to System Preferences - Users & Groups
- Unlock all options using the lock on the bottom left
- Click on the Guest User and make sure all options are off
- If all options were off, enable Allow guests to log in to this computer
- Then click on your primary username, then go back to the Guest User and turn off all options again. The Users & Groups dialog is really buggy. It might even automatically check Enable parental controls for no reason.
- Before you close the dialog, make sure that all checkboxes are unchecked and that the description of the Guest User says Off.
Step 2. Turn off all Sharing options/services (for now)
- Go to System Preferences - Sharing
- Uncheck all sharing options/services
Step 3. Disable iCloud Find My Mac (for now)
- Go to System Preferences - iCloud
- Scroll down the list, and uncheck Find My Mac
Step 4. Manually delete the Guest account
Open up a terminal, and execute the following command:
sudo dscl . delete /Users/Guest
Check that there is no user Guest anymore with:
dscl . list /Users
Step 5. Reboot
- Make sure that all dialogs are closed
If you are lucky, your guest account is gone!
If not, retry step 4 (the dscl command) and reboot again.
Once your guest account is gone, you can re-enable iCloud Find My Mac and your sharing options/services. But stay away from the Users & Groups dialog.