Finally resorted to manually killing it
<ElementType>CustomUpdateElements.ExecutableElement</ElementType>
<PathToExecutable>c:\Windows\System32\taskkill.exe</PathToExecutable>
<Parameters>/F /IM greenshot.exe /T</Parameters>
<ElementType>CustomUpdateElements.ExecutableElement</ElementType>
<PathToExecutable>c:\Windows\System32\taskkill.exe</PathToExecutable>
<Parameters>/F /IM greenshot.exe /T</Parameters>