My (little) phpbb2 board is readable and writeable by non-registered users. Registered users just have some more functions at their disposal.
I've dealt with spam adding a sort of very simple but personalized "captcha" (unregistered users have to digit a number they're shown).
That's because we want it to be a space of discussion open to everyone. We're not an elitarian community or a group focused on something in particoular. So, first you check if you like it, then if you plan to come back you register. I like Wikipedia style, so to say.
Then, it turns out that the ones who visit often also register.
Whatever I say, it's not my fault.