try it in vfp environment first and close the exe.see in the task manager if your exe stays here.
if ok (no exe in task manager),then:
second try it with your web app.i dont know how vfp is used here.Release all public refrences,vfp if object,...
as alernative solution you can build a bat file (DOS) with

taskkill /IM  your_vfp.exe  /f

for syntax can read https://technet.microsoft.com/fr-fr/library/cc725602%28v=ws.10%29.aspx

for on shutdown can read also this support example:
https://support.microsoft.com/en-us/kb/110970