You have complete control over comments made on your posts on your own WordPress.com blog.
If I (or anyone else) post a comment on YOUR blog and you do not want others to see it, you can UNAPPROVE that comment. This is different from deleting the comment, because you can still see it in your Dashboard.
To unapprove a comment, go to your Dashboard and click COMMENTS in the list on the left side. You will then have access to ALL the comments ever posted on your blog.
Roll over the comment you want to unapprove (or delete, or mark as spam, etc.) and then you will see the list of options. Each one is a link.
After you unapprove a comment, it is no longer visible on your blog, but you can still read it in the Comments list in your Dashboard.
It is NOT ETHICAL to EDIT anyone’s comments in any way! Please limit your actions on comments to approve/unapprove, delete, mark as spam, etc. NEVER open a comment and edit it unless the author of the comment asks you to do so (for example, maybe the person posted the wrong URL and sent you a correction).