That's how it works already. Turn on the column browser, display Genres, select a genre you'd like to remove, select all the tracks with that genre, get info. change the genre to something else. The genre will no longer be listed in the column browser.
There are, however, around 150 built-in genres that will still be listed in the Get Info. genre selection box. If you'd like to reduce this list further you can do so by editing the Localizable.strings file (in C:\Program Files\iTunes\iTunes.Resources\en.lproj or ...\en_GB.lproj for US or UK English). Back it up first and use something like Notepad++. Simply comment out the genres you don't want, and provided none of your tracks have those genres, they will disappear from the list.
tt2