Remove ‘Publish to Facebook’ checkbox for Facebook page publish plugin

That function is used to render button and it’s called inside fpp_post_submitbox_start_action(), which does some conditional check and is hooked into post_submitbox_start action.

First you need to remove plugins original action and use your own with conditional to render button. See code example below.

// Remove original action
remove_action( 'post_submitbox_start', 'fpp_post_submitbox_start_action' );

/**
 * Renders Facebook publish button, if post type is 'post'
 * and current user has 'publish_posts' capability.
 */
function mamaduka_post_submitbox_start() {
    global $post;

    if ( 'post' == get_post_type( $post ) && current_user_can( 'publish_posts' ) )
        fpp_render_post_button();

}
add_action( 'post_submitbox_start', 'mamaduka_post_submitbox_start' );

To remove check box only from specific custom post type you can change conditional post type check, like so:

if ( 'myposttype' != get_post_type( $post ) && current_user_can( 'publish_posts' ) )