Sounds like, there is an option of 'Remember password' that is in play.
If so, based on your requirement, it would be really difficult to force it on. Browsers today support storing username/passwords and that is totally on user. So, you can try this:
autocomplete = "off"
as:
<form id="loginForm" action="login.cgi" method="post" autocomplete="off">
I think this is IE/FF only... in case you still face issue and you still need something like re-login...
Use real
two-factor authentification[
^]to avoid the sole dependency on passwords which might be stored in the user's browser cache
Ex: having a compulsory checkbox to be clicked in order to login... or an image captcha to be read/entered in order to login.