At the moment there is no freature in 3cx (that I am aware of) that allows for this. Some ATA's do allow for outbound call blocking.
To make this happen in the current 3cx, you require a work arround.
I happen to have one.
Create a gateway (dummy gateway) make it what ever as long as it does not register.
Create an outbound rule [prefix you use to dial out]06
So when people select an outside line and dial a number starting with 06 it routes to you dummy gateway and nothing happens.