Obvious solution...
Google Voice already has the facilities in place for charging for calls (they provide "cheap" rather than "free" international calls): just pass these charges straight through, with a notification message beforehand telling you that ScamCo is charging you $1/minute to connect the call.
As long as the FCC doesn't require that long-distance companies charge the same price for these premium-rate calls as for regular calls (which would be truly insane), this neatly gets around the legal problem with "blocking" access, as well as getting some public awareness of this little scam, without leaving Google out of pocket.