Keep in mind, there are two different ways to manage Menus in WordPress. One is through Appearance > Menus, which is the screen you’re showing. The other is the Customizer.
Appearance > Menus : no coding required
You can prevent Posts from showing on a per-user basis. Just go to the top right corner of your screen, choose Screen Options, un-check Posts, and choose Screen Options again to close the panel.
Customizer
I’m not aware of a way to hide any registered post types specifically from the Customizer view. Hopefully someone with more Customizer know-how will reply if there is indeed a way.
Fortunately, this last section will remove Posts from both the Menus screen and the Customizer.
Changing a registered post type
When a post type is registered, there are arguments you can pass so that that post type will not appear in nav menus. Specifically, show_in_nav_menus.
Since Post is a built-in post type, you can’t adjust where it’s initially created. You’ll need to use register_post_type_args to allow you to adjust the already-registered post type.
<?php
add_filter('register_post_type_args', 'wpse_remove_posts_from_nav_menus', 20, 2);
function wpse_remove_posts_from_nav_menus($args, $post_type) {
if($post_type == 'post') {
$args['show_in_nav_menus'] => false;
}
return $args;
}
?>
This should prevent Posts from appearing in either the menu management screen or the Customizer.