Double book meetings

[Joe]

We have an nt4 exchange server 5.5 and outlook 2000 clients.

We have several conference rooms configured in exchange as a resource and
most users have permission to schedule their own meetings and delete their
own meetings in the resource calendar. What happens is users are able to
schedule appointments during the same times or overlap the times.

Is there any way to mark appointments as busy and to not permit others to
schedule during the same time?

Thanks,