I'm not really sure on the proper ettiquite, but I think it would be sufficient to note at the bottom of the invitation "no open bar" or "open bar not provided". Around our neck of the woods that would be suffient to indicate they would have to buy their own drinks.
I "think" that usually it is the other way around though; drinks are not considered included unless "open bar provided" is stated on the invite.
I've seen the phrase 'no host bar' used, which means the guests buy their own drinks at a separate bar, like at a wedding. If this is a sit down dinner where the waiter takes drink orders it would be different, and the bills would have to be done with separate food and drinks.
