Is it possible to use "be+not" contractions when giving orders?
She is not to be back late - She isn't to be back late
You are not to smoke here - You aren't to smoke here?
Possible, I would say yes.
However, I have always seen "She's not to be late" or "You're not to smoke here", mainly because this way, the negative (and therefore the prohibition) is stressed.
Please be aware that I'm neither a native English speaker nor a teacher.
"You're not to smoke" sounds odd to my ear. "You're not allowed to smoke" or "...supposed to smoke" or "...permitted to smoke" would be more natural to me.