If I delete a contact's email, are they able to re-subscribe themselves?

I am cleaning up our account and there are a lot of old email addresses (most of which are no longer valid).  But if I accidentally delete an email that is still good, and the person wants to re-subscribe, is there anything that prevents them from doing so?  In other words, if I delete a contact email address, is that a permanent deletion, that can't be undone even by the subscriber?