I used:
gsettings set org.gnome.desktop.wm.keybindings switch-applications "[]"
gsettings set org.gnome.desktop.wm.keybindings close "[]"
to disable Alt+Tab and to disable Alt+F4 respectively.
In Ubuntu 14.04: Alt+F4 worked, Alt+Tab failed (meaning the gsettings
succeeded but the short-cut is still active)
In Debian 8: Both Alt+F4 and Alt+Tab failed
How can I solve this problem? (Or is there another way to disable short-cuts through the command line?)