I want to create a secondary group that restricts access from the "off topic" forum on my site so I can stop "problem users" from constantly posting their bad political opinions, but I have found that creating a "banned from X" secondary group that precisely copies the normal Members group except for turning off access to a certain forum, simply does nothing.

Are permissions for people in multiple groups always the sum of all positive permissions from those groups? Is there no way to selectively restrict certain users from accessing certain places?

Sorry for the delayed response! There is a 3rd party extension that allows you to ban certain members out of certain forums which is essentially what you're seeking.