How can I disable wireless captive portal support?

Solution 1:

As documented in the Apple knowledge base:

If you cancel before logging in to the network

Tapping Cancel on the Log In screen disassociates the device from the captive Wi-Fi network.

If you joined the network from the Wi-Fi screen using the ⓘ button, or you joined less than eight minutes ago, you'll see a message telling you the network isn't connected to the Internet. You can choose one of these options:

• Use Without Internet: this option will dismiss the Welcome screen, turn off Auto-Login for the network, keep your device associated with the network, and allow you to interact with the network in other ways.

• Use Other Network: this option will dismiss the Welcome screen, disassociate your device from the network, and return you to the Wi-Fi Settings screen. From there, you can choose a different network.

• Cancel: this option will return you to the Welcome screen.

Therefore you can (now—I don't know in which version of iOS this was introduced) click cancel and then "Use Without Internet".

Solution 2:

To implement a Wi-Fi popup login page:
1) DNS request for www.apple.com must not fail
2) HTTP request for http://www.apple.com/library/test/success.html with special user agent CaptiveNetworkSupport/1.0 wispr must not return Success.

If you find a way to invert one of these result it shouldn't popup