AFAIK, that's pretty much the default. Guests can't post on any forum here, but can read all of them. That may have been a setting or option I changed way back when I first installed things, but at the very least, I know it's there. There's also a really advanced permissions system based upon groups, where you can pretty much allow/deny ANY action for any group or user. You could, for example, allow only certain groups to post polls, while allowing everyone to post regular topics, or only let people vote in polls in a particular board if they're in a special "registered voters" group. Pretty much anything you can think of.