How do you disable a 3rd party keyboard in your iOS app

Add this method to your UIApplicationDelegate

-(BOOL)application:(UIApplication *)application shouldAllowExtensionPointIdentifier:(NSString *)extensionPointIdentifier
    {

        if (extensionPointIdentifier == UIApplicationKeyboardExtensionPointIdentifier)
        {
            return NO;
        }

        return YES;
    }

Swift 5.1

Add to AppDelegate.Swift:

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

    return true
}