I'm sorry, but you can't

How can you tell someone that they are not allowed to do something? Pity whoever it was that had to tell the Portuguese prime minister that smoking on an aircraft contravened his own law banning smoking in public places. (The PM explained that he didn't think he was breaking the law as there was a curtain separating the first-class section of the aeroplane.)

If you want to tell someone they can't do something, start with an apology. This shows that you know what you are about to say might inconvenience the other person:

I'm afraid you can't…
I'm afraid you're not allowed to…
I'm sorry, but you can't / you're not allowed to…

You could even add "terribly" or "awfully" to stress this:

I'm terribly sorry Prime Minister, but you can't smoke here.
I'm awfully sorry to tell you this, but nobody is allowed to smoke on an aircraft.