unset form elements with hook_form_alter()
Notification does add its own form elements in its own hook_form_alter(), which is probably executed after yours.

Check the module weight in the {system} table, and set your module to a higher value to have it run later.

Note: Instead of using unset() to hide form elements, you should instead set the #access property to FALSE. This keeps them in the form build tree, which avoids problems if other modules try to access or alter that information.