Stop customers from buying New Customer only specials

[UrbanHeroesHiiT]

We are a German based fitness studio. Our Instagram and Facebook advertising takes people to our website where they purchase credits (passes) to attend our HiiT workouts, and Cryotherapy sessions. We often advertise special offers to attract new customers. Many of our current customers also learn of these special offers and then we lose money as they buy them also.

It seems that Shopify Scripts could be a solution. However, if our database of customers is kept in a separate Tech platform can the script then block the full list of our current customers?

What we want to achieve is that when someone goes to purchase the new customer special offer the Shopify form captures the First, Last name, Email and Phone. These four pieces of information are then used to look through the full list of our current customers. If the name, email or phone number matches a current customer then the Shopify checkout will notify them that they cannot make this purchase as it is only available to new customers.

Is this possible? Can Scripts do it? How do we get the Shopify side to have access to our full current customer list on the other platform so they will be stopped from buying/getting the new customer discounts?