How to change or delete the default Jazz Team Server login user id?

Is this the result of clicking the check box "Remember my User ID"  on the login form ?  If so, uncheck the form, remove your browser cookies with the name:  net-jazz-ajax-cookie-rememberUserId that are associated with the target server. Exit the browser and try reconnecting.  Most browsers can autofill fields or have a password manager that might be injecting the value.

If this is in Eclipse, go to the Team Artifacts view, expand the Repository Connections, right click on the one you'd like to correct, choose Properties.  On the next panel click on the Jazz Repository Connection and correct the user id.