"I was rebuilding my iTunes library to clean it up, and every time I went to change an album's genre to Rock, I had to look at Religious first. iTunes comes pre-configured with a long list of genres built in that I never use. So I decided to get rid of them. Here's how to do it. (Note that if this blows up your computer or insults your mother, it is not my fault!)
1. Navigate to /Applications » iTunes.app » Contents » Resources » English.lproj.
2. Open the Localizable.strings file up in a text editor (in my case BEdit).
3. Scroll down until you see /* ===== Genres ===== */
4. Comment out any genres you dont want. I commented all the genres out by placing /* at the beginning, and */ at the end of the list.
5. I also commented out all genres in the /* ===== Reduced Genres ===== */ and the /* ===== CDText Genres ===== */ categories.
Enjoy! The iTunes genres have now vanished from my iTunes library, and all the items labeled with custom genres have remained. As noted, try this at your own risk, as I really don't know if something might break."