Guests are already limited to what they can read. However, they still have access to 75% of the forums.
It would not matter, though, if we limited them even more. If they are going to read posts for 3 hours, it does not matter to them if they only have access to 3 of the sub-forums. Guests like reading old posts. Look at who is online and you will see that guests like to read posts that 4 or more years old. They hardly read the new stuff.