You cannot do this for the subscriber. Subscribers control their own subscriptions and we cannot delete, block or unsubscribe any subscriber from a public blog.
Here are brief instructions for your followers re:unsubscribing
1. They can click the “unsubscribe” link on the emails of the post they receive at the very bottom of the emails.
2. Or they can go here > http://wordpress.com/#!/read/edit/ and locate this:
“Get new posts by email” in the drop-down menu they have these choices:
3. They can also click the “x” next to any blog there to unfollow it and no longer receive posts in their Reader or by email.