Welcome credit abuse at scale

Finance flags that your new-user $15 welcome credit is bleeding money: redemptions are up 40x this week but real new-user retention is flat. Each redemption is technically valid, one credit per new account, per your rule. Investigation shows thousands of new accounts created from disposable emails, each redeeming exactly once, ordering cheap items to disposable/reshipped addresses, then going dormant. They stay under every per-account rate limit because each account does the minimum. Payment succeeds (prepaid cards). How do you triage the abuse, stop the bleed without harming real new users, and prevent recurrence?