...I'm really impressed with the number of people freaking out about this. You do realize that people have been cross-posting to facebook from LJ ever since facebook connect came out. You just weren't aware of it.
Anyway,
nakeisha posted this fix to disable the comment cross-posting.
1. Go to your "Journal" menu, and select "Journal Style".
2. To the right, you'll see what theme you're using, with a link stating "Customize Your Theme". Click that link.
3. Scroll down a little ways, and you'll see that on the left side of your screen, you have multiple options to modify your theme. Click on the "Custom CSS" link, and it should load the options for doing Custom CSS.
4. From here, all you have to do is go to the "Custom Stylesheet", and put in the following line.
.b-repost-item {display:none}
5. Once you have it pasted in there, save changes.
6. Feel free to share this with your flist.
Just thought I'd share.
(xposted to twitter)