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 section, and enable the "Only create commissions for a customer's first purchase" option.
Once ready to go live, hit the publish button.
Now, discount codes that are generated for affiliates in this program will be single use per customer!