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 and click on the edit icon of the program you wish to configure.
Scroll to the Advanced Program Settings.
Under Track Referral Sales of, select "Specific Customer Segments".
Choose "Customers who haven't purchased" from the drop-down list.
Once ready to go live, hit the publish button.
Now, discount codes that are generated for affiliates in this program will only be eligible for new customers.