Not entirely sure what you mean here, but I think you're asking if you can make it so that a certain group of users does not have email access.
If that is the question, the answer would be - no. You can disable web mail, but they can still get mail normally. I suppose you could play with settings making it difficult for them to retreive any mail, or set their disk quotas very low, but they could still get it.
If anyone knows another way of doing it, please step forward.