Yup. As long as they have the ribbon (usually Classic Ribbon, but some events give other ribbons, and I have a sneaking suspicion those are blocked too), can't Surprise, Wonder, GTS trade them away.
There are a few rare events with no ribbons (one of the Yokohama outbreak chus, if I'm not mistaken) and I'm not sure where does lie.
Additionally, while not completely off-topic, it appears HOME can't trade ANY Mythical Pokémon.
edit: From Bulbapedia's page: