how to prevent appointment acceptance if time slot busy

Is there any setting available to prevent creation of a calendar entry (or even better accepting an appointment invitation) when particular time slot is already busy?

Background of this request: Our office assistant maintains calendars for meeting/conference rooms. This means that she receives invitations when someone creates a new appointment and requests one of conference rooms. She would like to be prevented from accepting such an invitation and creating a new calendar appointment if the time slot is already reserved (i.e. it is "busy"). Is it doable?

This is Zimbra 5.

I don't think so. We've had the same issue. Is there a way that this can be added as a feature? The meeting should not be able to be booked unless the meeting room is free...

Here are a few of RFEs you may want to vote on:

10515 enforce non-conflict scheduling (Scheduling Wizard)
14293 Alert on creating a scheduling conflict
29849 Alert user when accepting a conflicting appointment (just added)

Unfortunately none of these has much interest as demonstrated by votes. :(

Well, those requests are different. I am fine with having double booked meetings for people. But for meeting rooms, this should not be the case. Someone can be double booked and then decide at the last minute which meeting they want to go to. A meeting room needs to be booked out definitively.

Most options are applicable to both users and resources. Your best bet would be to vote for 10515 and comment that you're interested in it for room scheduling.

And of course if you're a NE customer your can ask them to add a support case against this bug or create a new one saying that you need this functionality.