Delete all the files in "forum" folder or If you have other non MyBB files in this folder,you can download a fresh myBB from
http://www.mybboard.net/ and make a file list of the myBB files so you will remove only the myBB files.In DB delete all the tables with "mybb_" perfix.After that you can remove the integration files of YSE.Edit "forums.php" and "signup.php".You can also delete "include/mybb.php" in Yuna Folders.