How does one disable third party keyboards in Swift?

Add this code in your AppDelegate

func application(application: UIApplication, shouldAllowExtensionPointIdentifier extensionPointIdentifier: String) -> Bool {
    if extensionPointIdentifier == UIApplicationKeyboardExtensionPointIdentifier {
        return false
    }
    return true
}

Swift 4:

func application(_ application: UIApplication, shouldAllowExtensionPointIdentifier extensionPointIdentifier: UIApplicationExtensionPointIdentifier) -> Bool {
    if extensionPointIdentifier == UIApplicationExtensionPointIdentifier.keyboard {
        return false
    }
    return true
}

Apple reference.

Swift 4.2

func application(_ application: UIApplication, shouldAllowExtensionPointIdentifier extensionPointIdentifier: UIApplication.ExtensionPointIdentifier) -> Bool {
    if extensionPointIdentifier == UIApplication.ExtensionPointIdentifier.keyboard {
        return false
    }
    return true
}

Swift 3 :

func application(_ application: UIApplication, shouldAllowExtensionPointIdentifier extensionPointIdentifier: UIApplicationExtensionPointIdentifier) -> Bool {
    if extensionPointIdentifier == UIApplicationExtensionPointIdentifier.keyboard {
        return false
    }

    return true
}

func application(_ application: UIApplication, shouldAllowExtensionPointIdentifier extensionPointIdentifier: UIApplication.ExtensionPointIdentifier) -> Bool {
  return extensionPointIdentifier != .keyboard
}