Abuse account signup surge

Your signup dashboard shows new-account creation up 25x in 6 hours, all from a handful of cloud ASNs, using disposable-email domains and sequential usernames. These accounts immediately call your free-tier LLM completion API at the per-account rate limit, burning a large GPU bill, and a few are scraping your private 'people search' endpoint. Conversion to paid is zero. Your fraud queue is clean (no chargebacks yet) so finance hasn't noticed, but cost-per-hour just doubled. How do you triage and shut down this abuse without harming legitimate new users?