Say for some reason or another you've cancelled a Compressor job but noticed that your temp is still up (if you use something like SMC Fan Control) and the Mac is sluggish it's likely because Compressor has been told by Batch Monitor to keep compressing a file even though you've told it to stop, trashed the output file and anything else. The job's in the que and Compressor is determined to finish it in spite of you.
You'll even find that if you quit the job in Activity Monitor it'll just relaunch and keep on compressing. It'll even keep compressing across a reboot. What do you do?
Launch Batch Monitor (likely in your Utilities Folder, Hit Command + Shift + U to open that folder from the Finder) and hit the nice X stop button (in-between the Pause and Info buttons) to cancel the job. Ignore any complaints from Batch Monitor and just hit the X button again if it does complain.