If you have access to SendGrid, navigate to the marketing account and head to Suppressions > Global Unsubscribes > Search for the guest email. If you wish to re-subscribe the guest, you can click on the email found and mark for deletion.
Global Unsubscribes information can be found in SendGrid documentation https://sendgrid.com/docs/ui/sending-email/global-unsubscribes/
If further assistance is required, submit a ticket to Punchh Support. (For help submitting a ticket, click here)