If Safari is set to automatically fill in user names and passwords for you, you can turn off this feature and delete any user names and passwords Safari has stored.
Choose Safari > Preferences and click AutoFill.
Deselect the “User names and passwords” checkbox, and then click the Edit button next to it.
Select individual user names and click Remove, or click Remove All.
Passwords are not shown, but if you remove a user name from the list, its password is also removed.
Click Done.