This is a high level answer given the lack of specifics in your question. When the user selects a time, you should add the following to your OnSelect event of the gallery or button (whatever you use to let the user select the room).
//Refresh the datasource
Refresh(YourDatasource);
// Filter the data source looking for other events in this room
UpdateContext({RoomEvents,Filter(YourDataSource, RoomID=selectedRoom,Date=SelectedDate)});
// check if there are any items in RoomEvents. If there are, then the room is no longer available
if(RowCount(RoomEvents)>0,Notify("This room is no longer available",NotificationType.Error))
Again, this high level, but should get you going in the right direction. You should do something similar when displaying the list of rooms in the first place, filtering out rooms that are not available.
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