We recently went through this in a Grand Prix Legends racing simulation forum, with people allowed to post as "guests", which is absolutely and truly allowing anonymous posters. Over the last two years we had several really nasty situations develop with "anonymous forum vandals" posting crap, vilifying individuals, and going off topic to start threads about things other than our forum focus.
Several months back the moderators took the forum to a 100% registered mode, with NO anonymous posting allowed. Since then the only troubles we have had with forum-benders have been due to a couple of registered members who are great contributors but are highly opinionated and verbose. Know anyone like that?
I agree totally that totally disallowing non-registered posts is the way to go. Otherwise the moderators are forced to attempt tracking unwanted anonymous posts via the IP address, which is usually set by DHCP servers and therefore useless. Also, if only registered members are allowed to post the moderators' job is much, much easier.
If one of these "freebanders", as they seem to be called, registers and then begins posting CB crap or causing problems on the forum, simply send him a message that he is being banned from the forum and then delete his registration. They sometimes return under other guises, but usually one or two bans drives them to other forums where their posted subjects are more welcome.
Seem logical? It worked for us on the GPL forum....