For background on why non-customers can become referrers by default, see Understanding Non-Paying Referrers.
At an Offer Level
Follow these steps to ensure only paying customers can refer friends. 1. Click Campaigns. 2. Click the name of the campaign you wish to amend. 3. Click Drafts, Upcoming & History. 4. Click the experiment actions menu (three dots). 5. Click Duplicate (to draft) or Create New A/B Test. 6. Click the offer actions menu (three dots). 7. Click View Offer Settings. 8. Click the Auto-Decline Rules tab. 9. Click Add Rule. 10. Select the Referrer Must Be a Paying Customer option.
At a Campaign Level
You can block manual registrations for new referrers via the embedded registration page in the campaign settings. 1. Click Campaigns. 2. Click the name of the campaign you wish to amend. 3. Click Manage. 4. Click Settings. 5. Click the Advanced Options tab. 6. Click the checkbox for Block manual registrations from new referrers (eg via submitting the form on an embedded registration page).