Hey George!
There might be a few ways to do this, but if you are using the system exec VIs you will have to know if the application accepts any command line arguments. This may allow you to quit your application using the system exec VIs. A much better solution would be to make calls into the Windows API and close applications this way. There is a toolkit available that will allow you to do this, linked
here. Let me know if this works for you!
Hope this helps!
Andy F.
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National Instruments