When you disallow something, you prohibit it. Teachers usually disallow cell phones in their classrooms.
You're most likely to come across the verb disallow in official or formal contexts, like a list of rules in a courtroom or within the wording of a law. Referees often disallow certain actions in sports matches, and prison wardens disallow many kinds of behavior by prisoners. The word comes from allow, with its root allouen, "to praise, approve of, or be pleased with," with the Latin prefix dis in front, here meaning "do the opposite of."