its borderline harassment - but here are my thoughts.

offer them the opportunity to opt out of the mailing list - and if you get an email, make sure they are off of it. I would recommend monthly emails so as not to draw too much notice to yourself and piss them off even more.. most can handle a one a month - but make sure that in the email there are strong reasons for them to return - new content, hot action etc. create a feeling and family - remind them why they signed up in the first place