If you do accidentally connect to a network, before you realise this, there's no way to get it to forget. Rebooting doesn't work. The only solution I found was walking 30m away to the other side of the parking lot out of range of WiFi.
Start the install over from scratch. You can have wifi enabled when installing. Just don't plug in a network cable and don't connect to a wifi network when it prompts you to. Read carefully, you can avoid connecting and if you do you don't connect, you don't have to use a Microsoft account.