How to unassign Alt key from window context menu in GNOME

GNOME Connection Manager is intercepting the Alt key combinations for its own menu and refuses to pass it on to the shell it is hosting (where your MC instance is running). This is not configurable.

This is a bug in GNOME Connection Manager, or to be precise in the GUI library it uses (Gtk), so the only good solution is to go to their site and report the bug.

The other approach is filing a bug with GTK about "Alt being hardcoded" but you will need a lot of charisma and endurance to convince them to make Alt configurable. All alternatives will be workarounds, of which the ESC-key trick is the most elegant.