If it were me, and I wasn't willing to just block the traffic, I might just set a 128 kbps limit on it and call it a day[1]. Eventually, the other side will figure out that their fetchers are all backed up and work out how to do their job without burning so much bandwidth. [1] Yeah, that can be a bit of a pain to setup depending on the server settings, but some people have to pay for bandwidth and server resources, s…
It's a standard response code, and with a bit of luck google will scale the requests accordingly. If not, this will still allow the proxy to operate properly without burning too much server resources.
(I understand that this may leave some customers unhappy since the proxy may be up to 1 hour stale. If that's the case, the server could also add a check to see if there have been pushes since last request, though this is quite more complex)