Meeting room calendar: Overlapping appointments turn Tentative instead of being declined

We have a few meeting rooms, all of which have their own resource and calendar. In most of these resources, making an appointment on an already used timeslot results in the appointment being declined. One of the rooms however seems to add the appointment as Tentative. Because of this, users make overlapping appointments at the same time in the same meeting room.

Does anyone know how I can make this calendar reject overlapping appointments instead of them turning tentative?

EDIT: Solved by KavyaJune. See his comment for more details