Hi Nick,
I had the same issue- so i looked into how it could be done and discovered that it could be done with the 'killtask' command in a cmd window so I created a task to fully automate the process.
I know this thread was old but there is still no "End Process" tool within AA, and no public solution had been offered. So here's my offering...
See below the attached task. It may need adjusting according to your configuration, OS etc.
This task kills all excel.exe processes but of course could easily be adapted to end any other process required.
Kill_Process_(example used is excel.exe).atmn
Enjoy
H