at the top of the router there are 2 'push' buttons, one is a padlock (this is the WPS - WiFi Protected Setup) and the other is illustrated with a circle with brackets (((o))) this is the WiFi Switch. Press and hold it for 3 seconds and the light will go off. WiFi is now disabled. Repeat the process to re-enable the wireless radio.
You can also disable the WiFi from within the router setup page.
Open a browser --> Type 192.168.0.1 in the address bar --> press enter --> Username = admin, Password = changeme --> click login --> click Wireless Settings on the left hand side of the page --> Uncheck the box Enable Wireless --> click Apply