You can reset the user's password. When at the GUI login screen press ctl + alt + F2 . From there you can login as root with the root password. You will then need to set a new password for the user by typing:
You will be prompted to enter the new password 2 times. Then simply log out and reboot or ctl + alt + F1 or F7 to get you back to the GUI login.
Hope this Helps,