There are two options right now - require registration to comment at all OR require approval before publishing. I currently have all comments moderated but that slows conversations down. What would be good would be to allow registered guests to comment (without approval) AND let others comment but require their comments to be approved before publishing. Then if a registered guest spammed, they could have their privilege revoked. If a non-registered guest spams, their post would not get approved. IP banning is good but I suspect spammers have a large number of IP addresses to use.