When you want to post a new entry, there's that 'Crosspost this entry' option that you tick, right? Well, right next to it is the 'Crossposting settings' link, and it'll take you to 'My Account Settings'/'Other sites' directly. Under the title 'Crosspost Settings'->Footer, pick the 'Never' option for 'Add the footer to the post on the other site:'.

And there you go. =)