Selecting an entity reference once
I guess the Parking slot 1, Parking slot 2, etc. are nodes or fieldable entities. Most likely those nodes (or entities) have a boolean field which indicates if the parking slot is reserved already. For the sake of this example,
I call it field_reserved.

In this case you need to implement in a custom module a hook_form_alter().
I am going to write a pseudo code.

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.
}


Hopefully this gives you some guidance. 
If you need a complete snippet, you will have to provide more data about your setup, like machine name of the fields, etc.