If you put the viewing restrictions as to "Members of the board, mods and admins" then Non-board members will not be able to read or post in the given forum the setting is set for, no spidering is done on these semi-closed forums.
If you set the viewing restriction to User's Mod's and Admins' anyone logged into BbBoard with an existing account is in effect a non-member, but the moment they post in a forum marked with "User's Mods & Admins," they can see and post in the ones marked as above with "Members of the board Mods & admins." Because the action of posting in a "User's Mods & Admins" forum they are a member of your board now. You can have forums open for "Guest Viewing" but only offer posting restrictions to at least "Users Mods & admins" so the spammers without accounts can't post. But Google and others can spider pages for indexing on search string results.
I hope that helped.
No questions here are ever not worth asking so please feel free to post if you have other questions.