Question: How to disable categories/most used in 'add new post'?
I tried function: remove\_meta\_box(), but it doesn't work for me, i tried it in functions.php for my theme, it's mistake?

Answer: 
`remove_meta_box` removes the whole Categories box.

The `Most used` tab don't have any hook.

**Don't modify WordPress core files**, there's always a way to modify WP behavior without touching the core.

```
/**
 * Place the script in the theme's functions.php file
 */

add_action( 'admin_head-post-new.php', 'wpse_70874_hide_most_used_cats' );
add_action( 'admin_head-post.php', 'wpse_70874_hide_most_used_cats' );

function wpse_70874_hide_most_used_cats()
{
    // This function runs in all post types (posts, pages and cpts)
    // So we need to check for the correct type
    global $post;
    if( 'post' != $post->post_type )
        return;

    // If checking for a custom taxonomy, the <ul> id is 'custom-taxonomy-slug-tabs'
    // See /wp-admin/includes/meta-boxes.php, line 322, WP 3.4.2
    ?>
    <style type="text/css">
        #category-tabs .hide-if-no-js {display:none;}
    </style>
    <?php
}

```
In *Add New Post*, click on the *Screen Options* tab om the top right then uncheck *Categories*. **If you want to change it for all users**, you can either:

* sign-in as the user and then change their screen option
* edit your theme's function.php. there are a couple options [here](https://wordpress.stackexchange.com/questions/15376/how-to-set-default-screen-options) and [here](https://wordpress.stackexchange.com/questions/56606/how-to-remove-certain-screen-options-and-meta-boxes-from-add-edit-post-type) that might work for you
* try to see if this [old plugin](http://wordpress.org/extend/plugins/wp-global-screen-options/) still works with your version of wordpress