If your computer is a laptop: Look around the computer for a physical wireless radio switch (Most laptops have one). If you find one, turn it off, and your WiFi will stay off until you turn the switch back on.
(Please note that the switch usually disables all wireless funcions including Bluetooth, so if you regularly use Bluetooth or other wireless radios(eg for a Bluetooth wireless mouse/keyboard) you won't be able to use them with the switch off)
If you have a laptop without a switch or have a desktop with an in-built WiFi antenna then you may be able to switch off the radio through the BIOS. If you don't know how to do this, let me know (along with your computer details) and I'll be happy to teach you how. Good luck!
It's actually because of virus from a hotspot. Yes, it is laptop and has radio switch. My laptop start very slow and become slower if the wireless is on. I always turn the wifi off before I turn off the computer, but randomly it will on again if I restart the computer. I reinstall the computer and now everything back to normal.