Unlike Gnome - the keybinding and mouse binding is controlled by a configuration file created when you first login -
Open the file:
Just below this are the statements highlighted:
Alternatively just delete the lines below if you no longer want any ALT key mappings.
You should be able to move between desktops with ALT+SHIFT and Clicking.
You will also note if you search for
A-Down there are various other similar mousebind XML tag situations where Alt and click are used - you could also think about changing these mappings to be consistent.
Save the file, logout and login to test the new key-mappings