I don't know if this is possible but what would be interesting for the forum is to let anybody access it and read the posts (even when they use a spammer's ip) but block them from signing in or creating an account.

I for example use a proxy at work to just read the posts (I don't sign in), but just to stay up to date, and because of this restriction I can't any more (or I use a php proxy in addition to the web proxy to get access) and it's annoying. I have to use a proxy at work, otherwise a lot of accesses to the Internet are blocked by the company I work at (but strangely the access to some open proxys are not).