2xx is inappropriate because request is not accepted even if it's understood and processed as stated in the original post. Using 2xx code implies that the request is successfully fulfilled, i.e. in case of GET, requested resource is returned in body.
As I stated in previous comment, ISP that blocks the request in this manner can be understood as a forced proxy and therefore 5xx is fine (server's error as in remote error, not made by client, cf. 502 and 504 codes).