Is there any way of leaving an iMessage group chat?

I've been getting the "St Pancreas" train to college for two years and it wasn't until I went to see Big Brother live a few weeks ago that I realized I'd been saying it wrong all this time.

I still only get EDGE in Kent, England - only in very densely populated areas such as London and popular shopping centers do I get a sudden boost to 3G, which even then free wi-fi is usually available from near-by restaurants.