I was doing a little research into Machine Learning and Spam Detection when I came across this post about us Nigerians on stackoverflow:

I'm trying to fight the battle of users trying to contact other users and asking for money. They are typically from Nigeria or Ghana.

Several things I've tried are blocking these countries entirely upon signup based on their IP. I've also tried detecting similar message content and limiting the number of messages sent to 5 per day.

This is very shameful and annoying most especially to those of us working for international clients.
http://stackoverflow.com/questions/14753260/whats-the-best-way-to-detect-spam-in-php-messages