I'm looking for a wireless router (G or better N) with at least 4 ethernet port with the on/off hardware switch for the WLAN. So I need the router always on but with the possibility to switch on or off the WLAN.
If you don't want to simply turn it off and on from your browser (with the requisite menu-crawl), you might try slipping a couple of large tin cans over the antennae; as long as the "tin" cans are conductive (i.e., not plastic) and larger than the antennae, they will attenuate the radiated signal by a bunch.
My Toshiba Laptop have a physical WLAN ON/OFF switch in the front, but with wireless routers I haven't seen such a product yet. You can still use two separate units, I mean a router and wireless access point with an Ethernet cable between them.