To remove usernames and passwords in gmail screen

In Internet Explorer:
Tools menu => Options => Contents Tab => Personal Information => Auto complete button => Click the buttons “Clear forms” & “Clear passwords”

In Mozilla Firefox:
1. Tools menu => Options.
2. Select the "Privacy" tab & deselect "Remember what I enter in forms and the search bar"
Then in the "Private Data" section, click on the "Clear Now" button & select "Saved form and search history"
3. Click on "Clear Private Data Now" button.