How to switch off iphones wifi for good?

Settings > Wi-Fi > Wi-Fi off

It is designed to switch on again if it was turned off from Control Center but it should not if it is turned off in Settings. See: Use Bluetooth and Wi-Fi in Control Center - Apple Support

If it is turning on after you turn it off in Settings as described below from that article, contact Apple support. There is a Get Support link at the top right of this page.

If you want to completely disable Wi-Fi and Bluetooth for all networks and devices, follow these steps:

• • To turn off Wi-Fi, go to Settings > Wi-Fi and turn off Wi-Fi.