I find 'Require email address verification' - the best option (along with registration required to post) - I must get 4 or 5 attempts to sign up from bots - most have a .ru email address.
The only spam we do get is from genuine users - who sign up and then post a message to 10 different forums - sadly it takes a while to go and delete them. We need a box on show users posts that allow us to select multiple messages and delete them in one go
