You can't block or delete followers. Either make the blog private or accept that reality because there is no means of blocking anyone on the internet from access any public blog and submitting comments if and when comments are open.
We cannot block or delete subscribers from public blogs. Anyone and everyone can access public blogs and that's indeed what the public designation means. However, please read this about public blogs that become private blogs > Blog Privacy and Subscribers > http://wpcommaven.wordpress.com/2011/09/05/blog-privacy-and-subscribers/
Note that you have complete control over everything that's posted to your blog via comment moderation
For trolls, you can “blacklist” them at Settings > Discussion toward the bottom of that page. Enter their email address, their username, and if they included it, their website URL. That will automatically send them to moderation so that their comments do not appear on the blog. Do note though that if they are determined, they can use a different email address or username to get around that.
Also, do not put their IP address in the blacklist. IP addresses are no longer unique, and you might end up blocking legitimate visitors. In fact for over a decade only about 30% of us have IPs that trace back to any individual computer. Also, all they would have to do is go to a wi-fi hot spot, or connect via a different ISP and they could get around that.