Assuming that GV is on sp1 and the area code to block is 345 then:
Voice Services > SP1 Service > X_InboundCallRoute:
{(1?345xxxxxxx):},{ph}
Of course, this will also block genuine calls from area code 345. You may be able to refine it more if the calls come from a certain number range.
The blocked callers should get a fast busy tone and "busy here" response.