Why do want to kill it if you first let it run? Wouldn't it be easier to not run it?
So i've been having problems getting an unkillable process to just accept it's fate and die. I've tried every task killing program i can find and every one of them comes up with the same error as Taskkill: "There is no running instance of the task"
The process in question is Battle.net and the Battle.net helper, both of which continue to show up in the details tab of Task Manager. After an hour on live chat with Blizzard support, the only suggestion was that the process is somehow getting hung in a piece of hardware. After that i tried turning off my system and unplugging it for a few minutes. That seemed to work, however it has since stopped working.
Interesting note that when i tried " taskkill /im battle.net.exe /t /f" it returned saying "The process with PID 9196 (Child process of PID 9112) could not be terminated" 9196 being battle.net.exe, i tried killing 9112 and it just said that the process was not found.
And help with this?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic