Bot nets likely behind jump in spam

The more I looked at the spam problems I was experiencing, the more I noticed my initial theory was wrong. The same spam posts were coming in from everywhere from Asia, to Africa, Europe, the Middle East and South America. Blocking IP space isn’t entirely effective either. I’ve already got some ideas for real solutions I can implement on my end, but what about the source? Vulnerable machines are at the heart of the issue. Automatic updating has helped, but obviously falls far short of a solution.