Try ungrouping the trace, viewing in DRAFT mode, then selecting anything that is not part of the image. You will usually find parts of the background that can be safely removed.
There is the possibility that some of your image will disappear with this method, or worse yet, certain lines will get removed, making your final output sloppy.
There are other methods for achieving the same effect that might be easier - it depends on your final application. Sometimes power clipping or editing in Photo Paint provides better results. If you need a scalable vector image when finished, sometimes manually tracing is the best method. Depending on your version - X3 is supposed to have some new support for this exact application, but I haven't tried it with anything, so I cant comment directly on its effectiveness.