Instead of trying to detect and block it, just disincentivize it.
Most AI spam on social media is trying to exploit various systems intended to predict “good” content on the basis of a user’s past content by tracking reputation/karma/etc. They build up karma by posting a massive amount of innocuous content, then leverage the karma to increase the visibility of malicious content. Both halves of this process result in worse content than if the karma system didn’t exist in the first place.
Wake up.