Firstly, I should say I know very little about GV, so I'm working in the dark here. azrobert has given you a blocking rule, which should return code 486 "Busy Here". If that sends the calls to GV voicemail, then problem sorted.
This is a little trick if you wish the incoming calls to get ringing for 25 seconds, then get sent to GV voicemail. You do need to have an spX set up for SIP, real or fake.
Set up a fake voice gateway – I'm using vg1. All settings at default except AccessNumber. I'm using:
AccessNumber: sp2(192.168.1.10:5490)
192.168.1.10 is my local OBi address, but the port 5490 is not in use, so the calls get sent nowhere.
Vg1 does not need to appear in any other DigitMaps etc as you are not using it to dial out from.
Next set up:
Voice Services > SP1 Service > X_InboundCallRoute:
{(1?8(00|88|77|66|55|44)xxxxxxx):vg1},{ph}
Any calls coming in with toll-free CallerIDs will hear ringing for up to 30 seconds, then code 503 "Service Unavailable" will be returned. However, as GV voicemail connects after 25 seconds, callers will hear ringing for 25 seconds followed by GV voicemail.
I have no idea if this is of any practical use for anyone