First of: 'have be served' is not grammatical.
It would be: have to be served (or have been served, but that would mean something different)
1- Drinks have to be served in room A, but not in room B.
2- Drinks must be served in room A, but not in room B.
Be careful:
don't have to: it's not important, you do not have to do something; You don't have to go to school tomor