You should be able to make a fake hidden password box to prevent it.
<form>_x000D_
<div style="display:none">_x000D_
<input type="password" tabindex="-1"/>_x000D_
</div>_x000D_
<input type="text" name="username" placeholder="username"/>_x000D_
<input type="password" name="password" placeholder="password"/>_x000D_
</form>
_x000D_