I have a number of compilation CDs which contain songs I know I don't like. As there are so many CDs, I didn't want to selectively import songs, so I just imported them all. When I come across a disliked song while listening to a CD, I rate it as one star. On synchronising with iTunes, these show up in my Smart Playlist:
My Rating is 1
I then look at the playlist and it tells me what songs to delete. Unfortunately, you can't delete them directly in the playlist, so you have to note the song name, then search for it in the library and delete it from there.
Dave adds: If you hold down option and press delete you can delete the file from all playlists and remove from your hard drive.