Here's a hack that seems to work in Firefox and Chrome.
In Firefox, having a disabled text field just before the password field seems to do the trick, even if it is hidden (disabled: none)
In Chrome, it has to be visible though.
So I suggest something like this :
HTML:
<input class="password-autocomplete-disabler" type="text" disabled>
<input type="password" name="pwd">
CSS :
input[type=text].password-autocomplete-disabler {
position: absolute !important;
left: -10000px !important;
top: -10000px !important;
}