blocking and deleteing WordPress.com subscribers and commenters
There is no means of "blocking" or "deleting" subscribers to free hosted WordPress.com blogs with the visibility setting of "public".
Note that you have complete control comment moderation. http://en.support.wordpress.com/settings/discussion-settings/#comment-moderation If your blog is public others can, of course, read it. They can submit comments but you can Moderate all comments and choose which ones to approve or not.
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. 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 to individual conmputers, and you may end up blocking legitimate visitors, beacuse ISPs have been placing hundreds of us in the same IP block to save money for a decade or more now.
In addition it's an exercsise in futility as many have dynamic (ever chnaging IPs) and any troll can get another IP in no time as 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.
I suggest is to set your blog so that first time commenters are held in moderation until the first comment is approved, then from that point forward they will not need approval. Settings > Discussion. The next step beyond that is to set your discussion page up so that all comments are held in Moderation until approved.