You basically need to add autocomplete="off" attribute to the HTML <form> element. However, this attribute is not supported on the <h:form>. It's only supported on the individual <h:inputText> and <h:inputSecret> components.
So, this should do:
<h:form>
<h:inputText value="#{auth.username}" required="true" autocomplete="off" />
<h:inputSecret value="#{auth.password}" required="true" autocomplete="off" />
<h:commandButton value="Login" action="#{auth.login}" />
<h:messages />
</h:form>
The new autocomplete attribute on <h:form> is scheduled for JSF 2.2. See also JSF spec issue 418.
Or, if you're using container managed login with j_security_check which requires a plain vanilla HTML <form> anyway, then just add it in there:
<form action="j_security_check" method="post" autocomplete="off">
<input type="text" name="j_username" />
<input type="password" name="j_password" />
<input type="submit" value="Login" />
</form>
See also: