Make the Divi Button Unclickable on All Devices
To make a button non-clickable on all devices (i.e. desktop, tablet and mobile), you can enter:
Make the Divi Button Clickable on Mobile Only
To make a Divi button unclickable on desktop / tablet widths, but leave it clickable on mobile widths:
First, follow the instructions above to make the Divi button unclickable on all devices.
Then add the following:
into the button module's mobile option of the "Button Settings > Advanced > Custom CSS > Main Element" box, like so:
That will override the main setting for the option, allowing the button to work on mobiles.