Smoking is not allowed on these premises.

Smoking is disallowed on these premises.

Are both sentences fine? Which sentence should be used on a notice board?

Thanks.

Disallow is usually used to mean deny the validity of, as in "The higher court disallowed the lower court's ruling." For your purposes, I would say that "not allowed" is the better choice, though "prohibited" might be better still.

Peter

I've never seen disallowed used in this context. Trombo's prohibited or not permitted would be the normal choices.