Yes, it is possible for the recipients to opt their email address out of receiving Tell-A-Friend (TAF) messages. This will not opt them out of site-wide email if they are a Constituent. It is not possible to opt the email addresses back in to receiving TAF messages as a Luminate Administrator. If you have permission from the recipient to opt them back in, then please Contact Support and provide the solution ID. Luminate Support can opt them back in, however, it isn't possible to opt them out; the recipient must opt out themselves.