Laptop or desktop computer?
If its a laptop simply turn off the wireless hardware. Most laptops have a hardware switch on the top or side of the case. It also may use a function switch combination like <fn><f2> for example. Check the users manual or manufactures web site for help.
On a desktop PC open the Network and Sharing Center then click on Change adapter settings at the left. Highlight the wireless adapter then right-click and either select Disable or select Disable this network device at the top of the window.
I had to do that with my wife's HP All-in-One desktop machine in order to not connect to my home wireless network.
Don't bother looking for a switch on the desktop...