Yes, the entire space should be gone (in favor of the main column. Sadly many themes hide the side column but do not expand the main one.
From what you said in here you would like the sidebar to disappear and at the same time the main column to expand.
This is a perfectly legal request. To do so the only the reasonable solution would be to edit the single.php file that is the template for your posts.
The HTML of your single.php file is something we should not style from functions.php.
Much easier you would typically need to remove the <?php get_sidebar(); ?> part and the update the HTML classes to expand the main column.