Recordings->ready/available(or whatever it says, I can't remember)
Click one recording, just as you would when you want to watch it. On the popup there should be an option to delete the recording.
the only problem there, is if the file is being accessed, eg via comskip it wont tell you, it will delete the recording from the list but the file will remain. you can delete videos from my videos, xrecord and video arch (i think). my videos will tell you (by the fact the file/dir isnt removed) if the file was successfully deleted (you delete these via the popup menu).
If the video file is in use when you try to delete it (for example by Comskip) the Video Archive plug-in will display a pop-up dialog telling you that the file could not be deleted because it was in use.