Your iPhone or iPad automatically joins Wi-Fi networks you've previously connected to. You can now override this behavior, telling your device not to automatically connect to a specific Wi-Fi network. Your iPhone or iPad will remember its passphrase and other details, but will only connect when you choose to do so.
This option was added in iOS 11, and is particularly useful for public Wi-Fi networks that you don't always want to connect to automatically---especially if they require a sign in or have slow internet. What's even better is that iPhones and iPads will automatically disable the auto-join feature for Wi-Fi networks with patchy connectivity, too.
To change this setting, head to Settings > Wi-Fi on your iPhone or iPad.
Tap the blue "i" button to the right of the network you want to stop your device from connecting to.
Disable the "Auto-Join" slider here. Your iPhone or iPad won't automatically join the Wi-Fi network in the future, but you can return to the Wi-Fi screen and tap the name of the network to manually initiate a connection.
On iPhones and iPads running iOS 10 and older, you have to forget a saved Wi-Fi network to prevent your device from connecting to it automatically. This means you'd have to re-enter the network's passphrase and other details if you ever wanted to reconnect.