To avoid users using your forms, check the $_SERVER['HTTP_REFERRER'];
You can have a form not on your site get posted to a script on your site. just set the action="" to whatever form you want.
If you want to steal a form, include it such as
include 'http://www.other.site/form.html';
but you'd probably need to modify the action if you do that. This also has very bad security implecations and I don't recommend doing it.
Tom