The only way you can do that now is on the client side. For 802.11N, you will bond two 20 MHz channels only on the 5GHz side. So clients that support 802.11N and the 5GHz should prefer the 5GHz than the 2.4GHz. But it will not stop client to use the 5GHz band which I think you are trying to accomplish.
You can always seperate the clients by ssid and only allow certain bands.... like create an 802.11N ssid for only 802.11N devices and a 802.11bg ssid for all others.
-Scott
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