Create a page that’s to not be indexed or accessible via slug

Why don’t you use is_page(id of the page) check with php and redirect them off that page to the main url?

Example:

if (is_page(22)) {
  //php redirect them to the main url
} else {
  //normal template continues
}

EDIT:

Just to be sure, you can use is_page_template check.

if (is_page_template(templatename.php)) {
  //php redirect them to the main url
} else {
  //normal template continues
}