To eliminate an existing filevault "master password", delete the files 'FileVaultMaster.cer' and 'FileVaultMaster.keychain' in your computer's main '/Library/Keychains/' folder (not through "Keychain Access.app"). You should then be able to set a new "master password" in the "Security" preference pane if you ever decide to use "filevault" again. Note that if you do this while an account is still encrypted (ie. with filevault enabled), that account will not be accessible with the new "master password".
Non eseguite la procedura se avete ANCHE attivato il FILE VAULT!