And also for a follower it should be possible to choose for unfollow.
This is already availabe. When you are on a blog you click the follow blog link to follow and you click that same link to un-follow. http://en.support.wordpress.com/following/#unfollowing-a-blog
If you don't want to continue to receive the emails of new posts you click "unsubscribe" link in the email at the bottom. Or you click the Manage Delivery Settings link under Read Blogs to change your default email delivery methods for all the blogs you follow.