Disable possibility to place external calls per hone basis

Is it a simple way to enable /disable the possibility of placing external calls to certain internal extensions ?
Better explained: all the internally defined extensions should be able to call one another and to receive external calls.
A group of “unlucky people” should NOT be able to place external calls, i.e. dialing number starting with 0 (I have an outbound route for dialing a PSTN trunk speaking about 0|. )
How can I accomplish that ?

Thanks in advance,