the login form for the admin section of the user registration solution pack does not set a default value for the user name and pass.
If the form is being loaded with an initial value, this will be a browser issue, most likely when prompted by the browser at some point to remember the user name and pass for this domain, you selected yes.
test this theory by loading the login page in a different browser.
Most browsers offer a way to manage saved passwords, you can delete the password that is saved for the user registration domain.
