How do I stop iPhone 7S Plus with iOS 11.1 from automatically connecting to WiFi even after turning WiFi off?

When turning off Wifi from control center, it merely disconnects from the current network, but does not actually turn it off.

It should not reconnect to a Wifi network unless one of the following is true

• You turn on Wi-Fi in Control Center.
• You connect to a Wi-Fi network in Settings > Wi-Fi.
• You walk or drive to a new location.
• It's 5 AM local time.
• You restart your device.

Use Bluetooth and Wi-Fi in Control Center with iOS 11 - Apple Support

Yes, it's now supposed to. Control Center only disconnects your device from the last connected to Wifi signal. To turn off Wifi completely you go to Settings > Wifi and turn it off there. Or you can simply say "Hey Sir, turn off Wifi."

Pls turn off the wifi as you did on ios 10, just reaching inside Settings, wifi, on/off.

Turn it off in Settings.