How to enable / disable accounts for Filevault2 pre-boot authentication?
I just remembered that there is a command line tool (fdesetup) to manage Filevault. Strange, Apple removed the option in System Preferences.

To remove the user "username":

sudo fdesetup remove -user username


To add the user "username":

sudo fdesetup add -usertoadd username


For more details:

man fdesetup