function mymodule_form_alter($form, $form_state) {

  // Check if the form id is the id of your form.

  // Use entity query that selects the node ids  of the nodes that are reserved.

  //Store the result in some array (let's say reserved_slots array).

  // With a foreach loop, check each select option id against the reserved_slots array of ids.

  //If such option exists, unset it.
}