Force a post to draft if specific meta is invalid, in this case the start and end date/time.

force-draft.php

<?php
/**
 * For a post to draft if specific meta is invalid, in this case the start and end date/time.
 * 
 * @author Joshua David Nelson, josh@joshuadnelson.com
 */
add_action( 'updated_post_meta', 'jdn_force_draft', 10, 2 );
function jdn_force_draft( $meta_id, $post_id ) {
	if( get_post_type( $post_id ) == 'events' ) {
		$start = get_post_meta( $post_id, 'event_start', true );
		$end = get_post_meta( $post_id, 'event_end', true );
		if( ( !$end || !$start ) || empty( $start ) || empty( $end ) ) {
			// Update post 37
			$my_post = array(
				'ID'           => $post_id,
				'post_status' => 'draft'
			);
			// Update the post into the database
			wp_update_post( $my_post );
		}
	}
}