Disabling Wifi Access impossible

I use a TPLink TD-W8901G router to connect to an ADSL line and to route traffic to a Time Capsule in charge of offering a protected wifi connection. Henceforth I would like to disable the wifi connection to the former; but I am not able to do it, nor to insert a password and neither to stop publishing the SSID.
As a metter of fact when I click disable everything gets disabled and I am not even able to click save.

At present I am thereafter offering my free wifi to the whole building and that is not fine.

Thanks,
Fabrizio Bartolomucci

Update router’s firmware with the latest available. Also update your browser and JRE.

I already got there alone. But how do I update the firmware of it?

Download the firmware from the official site: http://www.tp-link.com/en/support/download/
and upload it via the web interface of the router.

Ok for the dowload, but what is the menu item able to install it?