If you research this there is some setting today that gets you to all N mode and shuts out G devices.

But have you used the new N routers and gear? I was not happy when I tried this and found better throughput when I configured my N router back to G.

Your mileage may vary.

As to blocking, if you can use OPENDNS on the router, such controls do that pretty easily.

More at http://forums.opendns.com/comments.php?DiscussionID=2542 and opendns.com
Bob