Stop registered usernames from comment impersonation

Hello there. On my blog I wish to allow users to register to “reserve” their username for commenting. Currently I get people impersonation other users by using the same username. I need to know if there is a plug to stop this so that registered users’ usernames can’t be used by anyone else and the user has to be logged in to use the username. I hope you understand what I mean.

If there is no plugin for this, I think this should be a main feature of WordPress as users can even use the Administrators’ username. Regards. Tom.