Give Powershell a go
Right click on pictures and choose open in new tab to get a bigger view of them.
Run up IOT Dashboard and under Devices, Right click on your Raspberry Pi and choose Launch Powershell.
Put in the Admin password when prompted.
It can take a few seconds to connect, but when it does you should get a command prompt.
The to list your network interface type
Then to Disable the Wi-Fi interface
The do another Get to check if it is disabled.
Should stay disabled after a reboot also.
Hope this helps.