If you want this in the right way, then you must prepare a Landing-page where an user must login with captcha and can claim tru Steam login, and even can check if is a Unlimited account with real activity... then you will avoid all that persons requesting keys for resell. That are Not SCAMM, are persons taking advantage of a promo, unfair or not, are not scammers.
Is not very hard. But you goes for the speed way: Claim tru emails, or tru Gleam... not very smart indeed...