I have been using gmail / googlemail for a little while now and also have had some other email account with gmail / googlemail that I no longer use anymore, but in the login screen there is a drop list of previous typed username/login details and I wanted to delete these but did not know how, until I did come across this posting Delete autofills.
Basically the fix is, when you are on the login screen and selecting the login details using the keyboard up and down arrow, if you select one that you want to get rid of press SHIFT + DELETE, that is it.
It is a very cool feature but not well documented, so I thought I would also give it some air.