you will find all the settings and history in
/Users/[youruser]/Library/Application Support/Apple Qmaster
A good thing to do is to quit Compressor before making changes in this folder, then use Activity Monitor and (if it's still running) kill the "CompressorJobController" process (which will also quit all other related CompressorX and qmaster processes). Then you can just delete the contents of the History folder in the location above.
Do you use the cluster functionality? When i tested with different cluster settings, i ran into the same problem, that every job would get stuck. This didn't happen in a non-cluster environment. If you don't use a cluster, i would suggest to turn off Apple Qmaster Sharing at all. If you do, the problem might be, that your cluster is setup incorrectly because it cannot find any services to render your jobs and sits there forever.
If you want to start from scratch and reset Compressor to its defaults, then you might try to delete the "Apple Qmaster" folder at /Users/[yourusername]/Library/Application Support/ and remove the preferences as well (see /Users/[yourusername]/Library/Preferences/com.apple.compressor... note: there are two files!). But make sure, that no Compressor/Qmaster related service is running. You should kill those processes using Activity Monitor or just restart your machine before deleting the files mentioned above.
Additionally, you might want to check the contents of /Users/[yourusername]/Library/Application Support/Compressor/History/ and remove any file there as well.