This is inside my /etc/X11/xinit/xinitrc :-

Code:
setxkbmap -option terminate:ctrl_alt_bksp -option compose:ralt
xmodmap -e "keycode 22 = BackSpace BackSpace BackSpace BackSpace"
By having these the ctrl-alt-del is disabled.

And to reenable ctrl-alt-del, I just do this :-

Code:
# xmodmap -e "keycode 22 = BackSpace Terminate_Server BackSpace Terminate_Server"
In fact I wrote a little script which allows me to do it from the LXDE perference menu.