Make it as quick and painless as possible. They should be able to cancel via their account page/settings or contacting you. You should respond well within 24 hours with a generic, very positive note.

We're sorry to lose you as a member! Your account has been canceled (effective immediately|will remain active until the end of your billing cycle). If you'd like to renew at any time, blah blah blah. If you have any feedback for us about our service(s), we'd love to hear from you.

You will be surprised at how many responses you get back from an email like that from people who like your service, but maybe just don't have the need for it right now, and from others who will tell you exactly why your service doesn't meet their needs or expectations. Exit surveys are an incredible annoyance.