To restrict your referral program so only customers who have previously ordered from you can send referral in your Magento store:
- Login to your Magneto admin.
- Go to System (top menu) > Configurations (sub menu) > Refer A Friend (left menu)
- Find the Require Purchase section and update the drop down box to say Enable to require customers to have made at least one purchase before they can send a referral.
- You'll also want to review the message to show to customers if they try to refer without having made a purchase from you previously. Go to CMS > Static Blocks > search "purchase" under Title > click on Refer A Friend Purchase Required Notification > edit the message > click Save Config.
Update the drop down box to say Disable to make it so anyone with a My Account on your store can send referrals.