It certainly can be killed. I run a simple batch script which kills every job on the specified media agent every morning at the same time. I have multiple locations where sometimes the tech forgets to insert a tape, and I don't want the job to run into the day when they remember the next morning. Here is the command for the batch file:
"C:\Program Files\CommVault Systems\Galaxy\Base\gxcmd.exe" -cmd jobcontrol -commcell net-com1.beazer.net -port 8401 -logonid "********************" -operation "kill" -cl "atl" -clientType "MA"
A few things to note: to get the logon id, you will need to save a job as a script - any job will do. I got mine from saving a backup job. Where mine says "atl" you will need to change it to the media agent where you want the jobs killed. If you have multiple media agents, just copy and past it multiple times and change the media agent name. Then all you need to do is save it as a .bat file and schedule it to run whenever you like. It's simple, but works great for me. Hope this helps.