You can force them to log in before they post. I have added this to my forum over the weekend thanks to another Snitz forum member. You can check out my forum to see how it works. Just try to add or reply to a topic and you'll see what it does. If you then logged in, it would take you back to the post screen and allow you to post. Here is what he did:
'#### [Require Login to Post - NJP addon] ############################################## if mLev = 0 then Err_Msg = "<li>You must be logged in before you can post to the forums." & _ " <ul><li>If you are a registered member and already have a valid userID and password, please wait 10 seconds to be re-directed to the login page.</li>" & _ " <li>Otherwise click on the register link below to obtain your very own UserID and Password. Registration is Free.</li></ul></li>" & vbNewLine