Incorrect Form Entry data is one of the annoying things faced by web users, Many people ignore it though but some won't. I have seen many people deleting their whole browsing data just to remove incorrect form entry data, It is very simple to delete the unwanted form entry data leaving the necessary ones(correct usernames).
All you have to do is Hold Down the Shift key and select the entries by using cursor keys, after selecting entry Press the Delete Button .
You Can see it in Live below (animated GIF)
--- Short URL for Sharing - http://bit.ly/TNinFormDdel ---