How do I empty the cart?
Are you sure you want to empty the cart? Or do you just want to limit the quantity of any product to one?

I recently had a similar situation. I added a hook_form_alter to hide the quantity so it can't be readily changed. There's not a specific FORM_ID you can use for an add to cart form because they get a hash added to them, so it looks like this:

function my_module_form_alter(array &amp;$form, FormStateInterface $form_state, string $form_id){
  if (strpos($form_id, 'commerce_order_item_add_to_cart_form_') === 0) {
    ... plus any other logic specific to your use case

    _my_module_specific_add_to_cart_form_alter($form, $form_state, $form_id);
  }
}

function _my_module_specific_add_to_cart_form_alter(array &amp;$form, FormStateInterface $form_state, string $form_id){
  // Doesn't make sense to buy more than one, so hide it.
  $form['quantity']['#type'] = 'hidden';

  ... plus any additional changes, e.g., I have this:
  // There's only one variation, so hide it. (
  $form['purchased_entity']['#type'] = 'hidden';
}