Your users are receiving the emails because they are subscribed to all blog posts in the blog.
If you wish to subscribe users to a particular section of your content then I recommend using blog category subscriptions.
As for your existing subscribers, their blog subscription cannot be changed by modifying the subscription form. An interface for managing subscribers susbcription to blog posts and autorespodners is yet to be implemented.
But right now you will have to edit the MySQL database of your wordpress databsae to stop this from happening.
1. Find the database name in the wp-config.php file in your wordpress installation directory.
2. Log into your web host's control panel and then navigate to phpMyAdmin.
3. Click on the the database found in step 1. Truncate the table named wp_wpr_blog_subscription. Run the SQL query:
Note: wp_ is a database table prefix. Most websites use wp_ but your installation may be using a different prefix. Replace it with whatever your posts, users and options tables are prefixed with.