There should be a small icon bottom right of your screen showing 2 monitors, when you're online it appears there all you have to do to disconnect is right click on it and the box that appears will give you the option to disconnect.
If it's the same set up as our modem router, if you go to the two monitors and right click, that will disable the modem. I find that a bit of a pain, as you have to go into control panel to re-enable it.
You will be better off doing what phono say's, just turn it off.
Sorry to hijack this thread (tell me off if I am) but is there a security issue by leaving it on when not on the internet? I am using AOL and I have to sign on to access the internet. Netgear wifi modem / router