You can make a normal (non-agent) user of this user by going to the
Configuration -> User Access & Roles page. Then click on the
Make user button in the users table for this user. If the user (which is currently an agent) is assigned to one or more ticket, then you'll have to transfer his tickets to a different agent (by pressing the second icon to the right of this button for another agent) before you can make him a normal user. After doing that you should no longer get this message when logging in.
