When you unsubscribe a follower they have the option to opt back in at any time.
When you block a follower they are permanently removed from your database without the option to opt back in.
Here's how to Block and Unsubscribe Followers:
In your My.Popmenu dashboard, selection Communications > Followers
Search for the Follower's name or email
Select the Follower > Click User Settings
Block or Unsubscribe the User: