I agree with Ran - you didn't write the code of:
myUtilObject.convertInputStreamToString
but im guessing you loop there over the inputstream with a buffer you predefined its size (maybe with BufferedReader?) - in this loop you should check the stop condition of your async thread - isCancelled() is a good example.
The loop should stop if the thread is canceled, i.e.:
String line = "";
StringBuilder total = new StringBuilder();
BufferedReader rd = new BufferedReader(new InputStreamReader(is), 1024);
while ((line = rd.readLine()) != null && !isCancelled())
{
total.append(line);
}