How to force an IntentService to stop immediately with a cancel button from an Activity?

Here is the trick, make use of a volatile static variable and check continue condition in some of lines in your service that service continue should be checked:

class MyService extends IntentService {
    public static volatile boolean shouldContinue = true;
    public MyService() {
        super("My Service");
    }

    @Override
    protected void onHandleIntent(Intent intent) {
        doStuff();
    }

    private void doStuff() {
        // do something 

        // check the condition
        if (shouldContinue == false) {
            stopSelf();
            return;
        }

       // continue doing something

       // check the condition
       if (shouldContinue == false) {
           stopSelf();
           return;
       }

       // put those checks wherever you need
   }
}

and in your activity do this to stop your service,

 MyService.shouldContinue = false;