Hi,
By "users of a Room mailbox", do you mean users who have permissions to access the Room mailbox? If this is the case, I don't think we can prevent users from creating appointments. This is because the ability to create meetings or appointments
is based the permissions you have on the mailbox. If you have enough permissions, you can create both meetings and appointments.
If you don't want users to create appointments, as a workaround, we can remove permissions of the users. When users wants to create a meeting, he/she can add the room mailbox as an attendee.
Regards,
Steve Fan
TechNet Community Support