On earlier versions of Ubuntu, the AltF☒ and Alt⇆ shortcut keys for switching between virtual terminals seem to have been disabled under X Windows, so it wasn't possible to trigger them accidentally.
I'm now using Ubuntu 16.04 with Cinnamon desktop environment, and the shortcuts are enabled even under X Windows.
I frequently trigger them accidentally while trying to access menu items or to go forwards/backwards in a web browser.
How can I disable these virtual terminal shortcut keys, or replace them with the versions that require Ctrl (e.g. CtrlAltF1 instead of just AltF1)?
(I've found several older answers that suggest a custom
xmodmap, but this seems to have no effect for me.)