Is there a way to clear the dropdown list in the "user name" field when logging-in to sites? i.e. Yahoo.com email

I go to yahoo.com , or other sites, where a login is required. In the "User Name" box, when I click on it, a list of previously used user names is listed (such as when you log-in as different users, In the past I have made errors in typing in names (or some user names may now be obsolete). I would like to be able to remove the unnecessary user names. I have cleared the history completely and to my knowledge removed all cookies, but the unnecessary users names are still in the list.