You can remove entries from the list using the following steps:
Select the "Username" field and make sure it is blank
Press ↓ on your keyboard, a list of all of the "remembered" entries should appear.
Using ↑ & ↓ highlight an entry you would like to delete.
Press shift+delete. (Note that you may need to use shiftfn+delete, fnshift+delete, on Mac or ctrl+ +delete (Ubuntu)on Ubuntu in some instances, like if the remembered entry is in the address / URL bar).
Repeat steps 2-4 until satisfied.
To clear login entries for a username/password combination, see another answer.