While it’s not possible to block someone from viewing a public website, you do have some options.
Firstly, you can moderate your followers and remove them. There’s more information about managing your followers and specifically removing them here: https://en.support.wordpress.com/followers/#how-to-remove-a-follower
You can also change your comment moderation settings. Here you can ensure that comments from specific users and / or IP addresses are sent straight to spam. There are more details on how to do that here: https://en.support.wordpress.com/settings/discussion-settings/