I was thinking about if I should use moderation to stop spammers, although I have not had any yet (just started) and I was thinking how about an option to moderate first 'x' number of post by a member?
Then after that no moderation.
In the version after 1.1 we will be including post moderation. There will be group permissions to for posts being moderated or not. They can then be used with post groups to achive the affect you mention.
Also CAPTCHA will be included in 1.1 which should stop automated bots from joining