Greetings,
This new feature would essentially be analog to the no-object-entry parcel flag that parcel owners can set.
It's use is simple..to prevent accidents, to make it easier to create safezones where nothing can get in or our that isn't group owned, and for creating safer little sandboxes without being dependent on the settings of the parcels around the sandbox.
It should follow the current seperation between common user and group setting like rezzing, scripts and no-object-entry have, and it should only be settable if the no-object-entry setting is also enabled for everybody or the group, to prevent abuse (catching of objects that come wandering into the parcel).
While this would probably introduce new internal needed checks on parcel leaving, I think it certainly would be worth it.
This is a sorely needed thing.