Here's what you do:

  • Click the Finder icon in the Dock
  • Click Applications
  • Double click Utilities
  • Double click Terminal

  • In the terminal window, type the following:

    sudo fdesetup remove -user AUserName

    You'll be prompted to enter your password.

    This removes AUserName from the list of unlock users. Next,

  • Click the Preferences icon in the Dock
  • Click the Security icon in preferences
  • Click the FileVault tab
    There should be a warning message that "Some users are not able to unlock the disk"
  • Click the "Lock" icon at the bottom of the window and supply administrator credentials.
  • Click the Enable Users button
  • Click the Enable User next to AUserName
  • Click Done