Have a look here for some options: http://www.slipstick.com/calendar/skedresource.asp

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Milly Staples [MVP - Outlook]
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After furious head scratching, Frump asked:
| I have multiple conference rooms that I want people to be able to see
| but not directly be able to add appts. to. If I set the Default
| permission to reviewer they are not able to create appts. with the
| room as a resource, but if I switch it to author then the people have
| the ability to go right to the calendar and create an event, something
| we dont want them to do. What is the solution?