Jmeter does not have a flag to do this automatically.
Generally, you have to clear the results with CTRL+E (in windows) or from the file menu, under RUN > CLEAR or RUN > CLEAR ALL
You might be able to write a beanshell script to clear the results everytime you execute the script.
http://www.javadocexamples.com/java_examples/org/apache/jmeter/samplers/SampleListener/