Several months ago we saw a large bump in forum spam coming from the India / Pakistan / Bangladesh region. I started banning IP ranges, but they hit from so many I got pretty frustrated and banned an entire /12 for Airtel in India. (For those that don't know, that's basically a million IPs.) Forum spam dropped significantly immediately. However, it also blocked a large number of legitimate users. So about a month ago I dropped the /12 IP ban. Then sure enough, a few weeks later the spam returned. Now it's back in force.
We have a couple strategies we're mulling over to reduce this, and I hope to roll some of it out over the next few weeks. I appreciate your patience while we deal with it. In the meantime, reporting spammy comments and topics helps us a ton to get rid of it quickly.