Only allowing referrals for a customer's 1st purchase is a great way to protect your store's margins. Here's how to do it:

First, head over to the Programs tab on your merchant dashboard and click on the program you wish to configure.

Click on the Advanced Settings tab in the program builder.
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Under Discount codes apply to section, select Specific Customer Segments.

Choose Customers who haven't purchased from the drop-down list in the Customer Segments field.

Once ready to go live, hit the Save & Exit button.

Now, discount codes generated for affiliates in this program will only be eligible for new customers.