you can run the console program, (LXTerminal), and use "sudo shutdown -h 0" from there.
or typing ctl-alt-f1 will get you out of the gui to a console login. log in as root and use "shutdown -h 0" or as pi or whatever user account you have set up and use "sudo shutdown -h 0".
i'm not sure about the pi but, generally, on linux you'll have 6 such logins available, using alt-fwhatever to switch between them. alt-f7 will return you to the gui.
"If I had known it was harmless, I would have killed it myself." - Phillip K. Dick, A Scanner Darkly