如何删除从类中添加的WordPress操作?

I'm trying to stop password fields being added automatically to the Theme My Login registration form.

Looking inside Theme My Login's /theme-my-login/modules/custom-passwords/custom-passwords.php file, I can see the password_fields function gets added to the register_form action at approximately line 38. For example:

add_action( 'register_form', array( &$this, 'password_fields' ) );

I tried removing that action with the following but it doesn't seem to work.

remove_action( 'register_form', array( Theme_My_Login_Custom_Passwords, 'password_fields' ) );

Can anyone see where I'm going wrong?