Blocking outgoing numbers

Just don't allow those numbers to be dialed in your dial plan. Or--if they can be dialed, create a custom context in extensions_custom.conf which checks the number against numbers you don't want, then rejects the call.

Create a bogus, non-functioning ZAP trunk;
Create an outbound route with the dial strings you want to block;
Assign the bogus ZAP trunk to this outbound route;
Place the outbound route at the top of the list of outbound routes.