Registered usernames are not reserved!

How is it possible that usernames of registered user can be used when you do no enforce people to register first, before writing a comment.

Posting on my blog is possible without registrating first. Only a username(postname), e-mail and a body message is required! When someone however registered him/herself as UserA, this name can still be used by others that are not registered.

I assumed that registered username were reserved and couldn’t be used anymore as a postname by none registered users! Im stunned!

Is there a way this can be prevented. Some check that if a certain username is registered it can’t be used anymore and raises an error when the comment is submitted?

Thanks,
Lucas