To help protect your referral program and prevent behavior outside of your terms of use, you can blacklist referral codes.  This will effectively block the click from a referral link (with that referral code) from redirecting a user to the destination URL set for the campaign.  Instead, clicks on these referral links will be redirected to an offer not valid page.

Referral codes are a short string of alphanumeric characters at the end of a referral link.  For example, the referral code for the PURL http://fbuy.me/EzcW is EzcW.
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You can manage the blacklist of referral codes from  Settings > Configurations > Referral Code Blacklist.  From here, you can add new referral codes to the blacklist or remove existing ones.