Change global keyboard shortcuts

Here, in Xfce4 Settings Manager or launch xfce4-settings-manager from terminal,

In Window Manager configuration, find the keyboard part, look for Window operations menu, and then hit on Clear button, which will remove that shortcut key, effects immedately

Go to Menu → Settings → Settings Editor, select "xfce4-keyboard-shortcuts", select xfwm4 → custom → <Alt>Space, then press "Reset property" button. Every other key combination can be set and unset there too.

Settings Editor is the general way to configure everything in Xfce4, though not user friendly.