nrgymover,
Either one would work just fine for clearing the message; although DND ONs' solution is easier to remember especially if the feature code has been changed. Also using the SPCL key (X) is not usually required, by default, unless the system is designed to not let you activate feature codes without it; this is a system flag. In this case you should be able to just dial the Feature Code for Cancel Message [Default: 368] while the phone is idle.
Thanks,
TE