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If you hit Command+B (or Edit Menu >> Show Browser), 3 columns will come up in your iTunes window: Genre, Artist and Album. I have found that this is the easiest way to condense and remove extraneous genres. Also, if you select more than one song (e.g. all the songs in one useless genre), and click File Menu >> Get Info (or Command+I), you will be able to change the genre for all of them at once.
For Composer, you might be on your own. As far as I know, there is no easier way to change them, other than selecting them all at once, getting info and changing them together.
Hope this helps.