Deleted user still appears on system startup screen
I had the same issue and solved it thusly:

Open Terminal.

sudo fdesetup list


This will list usernames and uuids of the users FileVault knows about (warning: I'm unclear on the exact specifics of privileges granted by this file).

sudo fdesetup remove -uuid [bad UUID]


This will remove the erroneous user from the list.  For some reason I had to reboot the computer twice to see the desired behavior (i.e. the only person asked to log in on system start is me), but it works fine now.

If for some reason you need to figure out users' UUIDs (the usernames should be in that file, though my deleted user had (null) for a username),

dsmemberutil getuuid -U [username]


will give you their UUIDs.