Most things that are added to WP are added using actions and filters using the following WP functions:
add_action()
add_filter()
Anything added this way can generally be removed with:
remove_action()
remove_filter()
BUT… you have to apply the remove after it has been added but before it is acted on. That takes groking through the code to know where it is actually added so you can apply it appropriately.
In the case of the WP-Members shortcode button, the action is defined during the admin initialization. WP-Members has an action hook after this process – wpmem_after_admin_init – that you can hook to in order to remove any actions added during initialization.
The following added to your theme functions.php will remove the actions that add the button (thus removing the button):
add_filter( 'wpmem_after_admin_init', function() {
remove_action( 'load-post.php', 'wpmem_load_tinymce' );
remove_action( 'load-post-new.php', 'wpmem_load_tinymce' );
});
