I tried to keep out robot generated blog spam from this blog and never suffered a problem with comment spam. Recently a huge crop of manually-entered spam has overrun this blog and I basically deleted all the comments from Dec 1 onwards.
I bet Mechanical Turk is behind it, someone paying a penny or two to comment on blogs. Naturally everyone puts links in the comments which remain plain text and are useless and also puts links in the link field, which is nofollowed so not terribly useful either. Clearly the spam generators don't know that and don't care.
The question is: how do you keep valid comments in but manual spam comments out? I thought of asking basic programming questions but I think if you don't have a lot of them, the comment spam director would simply mention the answers in their request for spam.
Watching by IP may not be useful if it's a Turk crowd, since they will come from their own computers. I hate the idea of blocking comments from certain parts of the world.
Any (non spam) ideas?