Does Ctrl+Alt+Backspace not kill X any more (assuming you’re using X)?
Does Ctrl+Alt+Delete reboot the system from a graphical desktop? Or is that only from the virtual consoles?
I wonder if locking the session would have stopped it as well. I doubt the Alt+SysRq combos would have been useful since other random input was happening at the same time (unless the next keystroke happened to be an I, U, or B).