In an attempt to make my forum more uniform in the way that it looks, I'd like to block font formatting - specifically the abilities to change the font face itself, the font size, and the font colour.

Is this possible?

Sadly not: https://community.invisionpower.com/4bugtrack/active-reports/not-possible-to-deactivate-wysiwygtext-formatting-per-usergroup-r6420/

You can remove the editor buttons and force paste as plain text but there is no way to disable BBCode (font, color, size).

Thanks for the straight foward answer. Really wish this was possible.