1) ACP > Nodes List > Click "Permissions" next to the Category node I want to hide.
2) Im presented with a list of usergroups instead of Node Permissions, so I assume that I have to click a Usergroup, and there will be node-specific permissions in there.
3) As expected, Im presented with a list of permisisons, and i see the one I want: "View Node".
I make a mental note that it says 'Node' not 'Nodes' - so i assume surely this means only this Node will be hidden from this usergroup.
I log out, and see that the entire forum is now hidden.
So it totally didn't do what I expected. However - when I instead click on a Forum Node (sub-node of a Category Node) it does exactly what I describe above. Im presented with a node permissions page that is explicitly for the forum I want to hide.