Thanks robc. It actually disables ctrl, which is not very feasible during a session, but is ok when xtrlock is in use with Xvesa. If I remember as I don't often use Xorg, Xorg by default is using the "nozap" mode, which makes ctrl-alt-backspace not kill X.
I am not sure if there is a way to disable ctrl-alt-backspace when used as a combo without disabling those individual keys.