When I'm booked, I try to break it very gently in the nicest way possible. I'm always apologetic and tell them that if there were any way I could possibly take another order I would. If they push or offer extra money, I tell them that I have promised myself no more pulling all nighters and that's what taking another order would cause. That keeps people who are so inclined from taking it personally.
On the phone, sometimes it just a takes a minute or two for the news to sink in - not everyone processes bad news at the same speed.
But you can't please everyone and sometimes people are going to go nuts on you - just goes with the territory.