I've upgraded my LJ style for the sole purpose of "disabling" crossposting to LJ and Facebook, although I'm pretty sure you can work around it by using your own style when viewing, or similar things.
That said, I ask people not to do so. I do not use Twitter or Facebook, and if I want to start posting things there again, I will do it myself. Please do not cross-post (especially friends-locked posts) elsewhere.
I'm aware it's the internet, and Ctrl-C Ctrl-V is easy, but I can request it anyway.
Thanks to faiarrow for posting about it, and dydan for sharing a (at least partial) workaround.