Deactivating keys in a keymap


i search for solution for deactivating some keys in a keymap. I have done created my own keymap and i have edit it so i only can use the keys 0-9 and A-Z.

I need to deactivate the combination Ctrl+Alt+Delete so no one can open the task manager.

Can i deactivate this combination or is it a unchangeable system option?

Greetings Lelldorin