It's your right to run your blog as you see fit but please only take this drastic step after exhausting all options.
1. To ban a commenter you must first get the email address that is attached to their user name. To get this click on "Comments" under the dashboard and locate the comment from the user you want to ban.
2. Scroll your mouse over the comment area and you will see "Unapprove | Reply | Quick Edit | Edit | Spam | Trash" appear at the bottom of the comment. Select "edit."
3. There will be an area called "Email" in which the users email address appears. Copy that email address.
4. On the left side of the Dashboard select the word "Settings" and then under that select the word "Settings" again. You will now see an area in the middle where it says "Comment Blacklist."
5. Paste the email address of the user you want to ban into the "Comment Blacklist" area and select "Save changes." Each email address must be on its own line for this to work.
6. The comments from the user with that email address will still go through but they will go to the "pending" queue and will only become visible with your approval.
This method only blocks the email address, it doesn't block a person. So keep in mind that a determined person may create a new account with a separate email address.