I know some people who've said they've gotten nothing out of Last.fm and scrobbling, but I'm definitely not one of them -- Last.fm has had a HUGE impact on me. I've made a lot of really close friends (and I don't just mean "internet" friends, but "see-in-person," life-long friends), have expanded my music-listening considerably by discovering a ton of new bands, have discovered and attended a lot of concerts I wouldn't have been aware of, have learned amazing new things about the bands I was already listening to, and so much more. I highly recommend it to anyone who's a major music fan -- though your mileage with it may vary.
Well go back to preferences, click on "Scrobbling," and uncheck the box that says "Launch Last.fm with iTunes." That should do it.