Here's a Greasemonkey script that will disable those ()*&)(*&)*(& Facebook/Twitter crosspost boxes on livejournal.
Also, I will not be crossposting, if anybody wondered.
(Edited to clarify: it just takes the option out of your browser: it does not prevent other people from crossposting comments. Though why anybody would want to read a crossposted comment out of context is a mystery to me. Oh livejournal, try not to suck so much, please?)