I'm going to assume you mean "comment" and not post because the user would have to have an account to post an actual post on your blog and in that case you could easily set their level so they couldn't write posts.
As far as stopping people commenting using your blog owner's name, I don't really know of anything that can stop a person from using whatever name they want. (There might be something, though, and if so I'm sure another person will reply to correct me.) But you can force commenters to need to be registered to comment. They can still have the same display name as the blog author, but you can easily show the difference between a commenter and your blog owner if that's the case.
There are also some styling things you can do that format your comments so people can visiually see this commenter isn't the blog owner -- enabling gravatars, for instance.