How to kill a callback?

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I'm having some trouble with killing my callback function properly.

I have a main node (with its own ros::NodeHandle nh) which calls a class which initializes the following Subscriber with a ("/camera/image_raw"):

ros::NodeHandle nh;
int num = 0;
image_transport::ImageTransport it(nh);
image_transport::Subscriber sub = it.subscribe(topic,  1, imageCallback,this);
ros::spin();

The callback function in question is approximately as follows:

void imageCallback()
{
      num++; //increments the global variable num
      if(num < 10)
      {
           cv::imshow("view", cv_bridge::toCvShare(msg, "bgr8")->image);
           // ... do some processing using cv_bridge and write to a folder directory 
          cv::imwrite(name,cv_bridge::toCvShare(msg, "bgr8")->image);
          // ... the intention is that only 9 processed images are stored in this folder
          //       under the given name

      }else if(num >= 10)
     {
           sub.shutdown();  //  Intended to kill the subscriber once the 9th iteration is completed. 

     }
     cv::waitKey(30);
}

My problem is that once the value of num reaches 10, the main thread appears to halt and ROS ceases to print anything to the console.

However, if I were to remove the line sub.shutdown() , then we are still stuck inside the callback forever and we have no way of returning back to the program-flow of the main node.

How can we effectively "kill" the callback loop at its nth iteration and return back to the original node which called it?