Apple - How can I disable wireless captive portal support?

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".