Perhaps if the new post CGI were to:
• reject any new topic post with 25% or more content that is nothing but links to external sites
• lock user from posting for 24 hours if such a new topic post was made
It's unlikely that spammers will post during the day in the western hemisphere, because that's when it's nighttime in Asia.
I am on the west coast of North America and I routinely see a lot of spam show up in the late afternoon/early evening.
Those are not the only times the spammers do their thing, it seems at least to me to be a popular time.