Could you explain a bit more about what you are trying to do?
If you are trying to block a particular user by username, there is a link for that if you open one of the user's messages; look under the "Action" button. You can report the user to eBay or block the user. Or you can add the username to your blocked buyer list and set that to block messages from users on your list. That will not prevent messages if you are already involved in a transaction or if you respond to messages, but it may prevent the user from using the link in an item listing to contact you.
If you want to generally make it easy for interested buyers to contact you, there is no simple way to prevent other users from using the exact same method to contact you as well, though, unless you specifically block them by username.
