That's been our tactic for quite some time, but it isn't enough anymore. It's not individual netblocks that are the issue: the major spam factories have custom software that rotates their connections through multiple networks (and through multiple browser builds, to avoid browser-based fingerprinting) after every new account in order to evade automated blocking and detection. The big social media properties can afford to have engineers working full time on adapting to the tactics they're using, but we can't; geoblocking is the next tool we have available in the arsenal (for this particular problem; there are about six running concurrently) that has the least interference with legitimate use over the more invasive options.