This is not module or Joomla related. You cannnot control the browser from the webpage.The fact that the browser is filling the information is due to the user's actions of saving his password (Browser asks "Do you want to save password?). Doing this on a public computer is in deed not recommended but if a user chooses to store his information on a public computer, then it's neither your or our problem. This is common to all logins. Even Yahoo's mail login behaves the same. If you login to yahoo and save your information in your browser, every time you visit yahoo's mail the browser will fill in the information for you.

Conclusion:
1) This in not joomla or module related
2) You cannot control user activity or browser from code
3) You cannot _base_ your security on public computers (cpanel webmail advises everyone to LOG OUT every time they leave their computer)
4) You should advise (a small memo on the bottom of the module) your users that saving their passwords is only recommended on PRIVATE computers and they should logout as soon as they have finished writing their work