Cannot Resubscribe Message

Hi.
We have someone who unsubscribed, but would like to resubscribe themselves. If they go back to our website and try to resubscribe using plugin they get the following message:
You have opted out of our newsletters and cannot re-subscribe.

Is there a setting that would allow them to resubscribe or some kind of preferences center that would allow them to join again. How do we handle this??
Thanks so much in advance.
Mark

https://wordpress.org/plugins/constant-contact-api/