You could try setting the access permission on the Boards inside your two Categories so Guests (Visitors) can't see them.
Admin > Forum > Boards > Modify Boards > Select "Modify" on the board you want to hide and un-check Guests under "Allowed Groups:" and hit save, logout to see if it worked.
Further permission settings can be set per board where instead of pressing "Modify" as above, instead clicking on "Permissions".
Hope that helps.