Sorry, I don't know how to do that. I can see the XML file for custom ribbons in the UI file here:

C:\Users\<username>\AppData\Local\Microsoft\Office \Word.officeUI

...and I can manipulate the boolean value there, but I don't know how to do it with VBA except to open/modify the .officeUI file externally, but it's already loaded when Word is open, so changing it has no effect. My guess is, to change it while Word is open, you'd need to programmatically remove the button, not change its visibility attribute, then add it back when Word is closed. Someone on this board will know how to do that.