Could you please send your workflow and maybe some log for the job? If the process is indeed started by ffastrans it should have been killed by force. But I have seen cases where some process is impossible to kill. We're using the Windows API for such operations and if it cannot be killed by normal means it must be caught in some kind of deadlock or whatever that keeps it hanging in there. Unless this is the normal behavior we might have to put it on the Windows being Windows account.
Sorry also i have overseen this topic completely.
"You cannot kill the process" doesnt happen very often. Do you know if the hanging ffmpeg process was using GPU resources e.g. Nvidia encode or Cuda filtering?