That's not possible. Our stats do not report IP addresses to us. Given that IP addresses are no longer unique and haven't been for over 10 years that's not surprising. ISPs place hundreds of us in the same IP block to save money.
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/
To change blog visibility to Private go to Dashboard > Settings > Reading scroll to Site Visibility and choose option 3
Note that you have complete control over everything that's posted to your blog via comment moderation
Also note that these matters are not discussed on these public support forums that are available to all with internet access. You need to file a complaint and submit it to WordPress.COM Terms of Service Staff. Please read these so you know how to proceed: