This would need to be added to Serviio's configuration API before I could add it to ServiiDroid.
You are welcome to file an issue on Serviio's Issue Tracker maybe 'API action to kill active transcoding jobs', and if it gets added to Serviio I'll add it to ServiiDroid.
Just create a plugin which issues the command, similar to the refresh plugin described in the wiki, with a taskkill item, and play the item when you want to taskkill
Just to let you know, if you kill ffmpeg without Serviio knowing it, it'll end up in an inconsistent state and playback might fail (especially if you try to play the file that was being transcoded again)
Petr Serviio developer / site admin Do not send me PM for support as the solution can't be shared with others.