Downgrade user from role that allows post creation

Question

I am an admin of a multi-author blog. I am implementing a monitoring system which will require me to downgrade a user from an 'author' (this role allows them to make a custom post) back to the 'reader' role if I think it is a spam account, or if they are breaking website rules.

After changing their role using the admin screen, how can I make all the posts they made delete automatically, without having to scroll and find them myself?

Many thanks

EDIT using advice from answers below:

add_action( 'set_user_role', 'wpse98904_remove_demoted_user_posts', 10, 2 );
function wpse98904_remove_demoted_user_posts( $demoted_author_id, $role ) {
 // In here you'd search for all the posts by the user

 $args = array(
            'numberposts' => -1,
            'author' => $demoted_author_id,
            'post_type' => 'custom_post',
            // So if your custom post type is named 'xyz_book', then use:
            // 'post_type' => 'xyz_book',
 );
 $demoted_author_posts = get_posts( $args );
 foreach( $demoted_author_posts as $p ) {
     // delete the post (actually Trash it)
     if($role == 'subscriber') {
     wp_trash_post( $p->ID);
     // To force the deletion (ie, bypass the Trash):
     // wp_delete_post( $p->ID, true );
     }
 }
}

I used wp_trash_post to trash the events because adding 'false' to wp_delete_post did not work for me.

Answer 1

You can add actions to the set_user_role hook:

add_action( 'set_user_role', 'wpse98904_remove_demoted_user_posts', 10, 2 );
function wpse98904_remove_demoted_user_posts( $demoted_author_id, $role ) {
     if( 'subscriber' == $role ) {
         // In here you'd search for all the posts by the user
         $args = array(
                'numberposts' => -1,
                'author' => $demoted_author_id,
                'post_type' => '{your custom post type name}',
                // So if your custom post type is named 'xyz_book', then use:
                // 'post_type' => 'xyz_book',
         );
         $demoted_author_posts = get_posts( $args );
         foreach( $demoted_author_posts as $p ) {
             // delete the post (actually Trash it)
             wp_delete_post( $p->ID );
             // To force the deletion (ie, bypass the Trash):
             // wp_delete_post( $p->ID, true );
         }
     }
}

Reference

set_user_role hook -- Codex

set_user_role hook in WP Trac

wp_delete_post() -- Codex

Answer 2

Users roles are changed by the WP_User object firing the set_role() function. At the end of that function on line 815 of wp-includes/capabilities.php there is an action to hook to: do_action( 'set_user_role', $this->ID, $role );

So, in your functions.php or in a plugin, you can grab that data as the hook fires after the user capability update, and delete all of a user's posts with wp_delete_post.

add_action('set_user_role','myfunc',10,2);
function myfunc($user_id,$role) {
    if($role == 'subscriber') { // or whatever you want
        $posts = get_posts('numberposts=-1&author='.$user_id);
        foreach($posts as $post) {
            wp_delete_post($post->ID,true);
        }
    }
}

Now, be careful, because as is, this snippet will permenently delete the post. If you want to just move it to trash, change the second parameter or wp_delete_post to false.