What browser are you using?
With Firefox, click 'tools/options/security'.
Look around there and you will see options to review all username and password information that is currently stored on your computer by firefox. You will be able to display your actual passwords, highlight entries and remove them if you no longer want them stored.
(I rarely use Internet Explorer, but I think this info is OK)
With Internet Explorer, when you first get to the site, you will see the text boxes that ask for your email and password information. If the required information is already displayed there, highlight it and delete it. If it is not automatically displayed, then when you click in the empty text box another window will pop up and show your log in information. Highlight this and delete it. (You may need to 'right-click' after highlighting.) Another window should now come up and ask if you want this information deleted from storage. That should remove it.
Once the information is deleted, the next time you access this site you will need to supply the log-in information to proceed. The browser will probably ask if you want to store this information. Just say NO.
I hope this is accurate and helpful.