As was already mentioned disable or pull the wireless card from the PC.

Another option I did not see mentioned was adding a MAC address filter on your router to not allow that particular wireless card to connect to your network. To me that seems the preferred option as the wireless card is still there and active in the event you want to connect to yours or any other wireless network. You'd just disable the MAC filter from another computer if you ever need to connect (or just plug in the ethernet cable of course).

Dan
On-Wall M5HP LCR, QS8 & EP500 in 7.1