AC's suggestion is the easiest way, even if it means a fight with whoever is using the wireless connection to get your hands on the dongle ! When you want to permit access again, just plug the dongle back into the machine.
But it can be done via the settings page as well ... On my Netgear router, there is an option to set up a wireless card access list. Chances are there is something similar on your router.
You need to know the MAC address for the dongle and add it to the access list possibly with a name for the machine too), and make sure access control is enabled. Then when you want to block the other machine, delete it from the access list, and add it back on when you want to permit access again.
It doesn't take long to do the add or delete bit, and I use it sometimes here when the girls are being difficult and I want to chase them up to bed with minimal fuss or stop the oldest one using her laptop until the early hours ("oh dear, there's a problem with internet access again <sigh>") :-)