I've downloaded hundreds of songs from SXSW 2009 Showcasing Artists. In Songbird, I've filtered out a lot of songs which I don't like, i.e. gave rating of one to them. I hope to delete all these from my hard disk, but it's a feature yet to implement in Songbird. The most close solution I could get is this. But it works not well, and always messes files around. So, hope you could bring up a reliable and better solution.