I got very annoyed by the ALT shortcut opening Unity HUD. It was driving me crazy till I disabled it.
Straightforward solution
Open System settings.
Click Keyboard settings.
Open Shortcuts tab. Select Key to show the HUD and press BACKSPACE.
Alternative solution
Use the following command to disable Key to show the HUD shortcut.
$ gsettings set org.compiz.integrated show-hud "['']"
You can restore this shortcut using the same command with modified parameter value.
$ gsettings set org.compiz.integrated show-hud "['Alt']"
Simple as that.