Thank you for coming back to me Macropod.

I had looked at the formatting restrictions on the Review tab and in the Manage Styles options. Neither of these restricted (greyed out) the use of the 'Paragraph Spacing' button on the Design tab. I wanted to make sure that an inquisitive end user didn't change the document defaults, which could impact other styles (although I've set the important ones to 'no style' and set their line/paragraph spacing', so it shouldn't).

The only way i could think to do it is to remove the button from the Ribbon.