Found this at geeklikewoah 

You're probably all aware that Livejournal now allows you to repost comments to Facebook and Twitter. This is a very unpopular new feature. Firefox has an addon that hides the new Facebook / Twitter repost ticky boxes. But here's a little line of code you can use to make it so that no one, no matter what browser they're using, can repost comments.

.b-repost-item {display:none}


Here's what to do:

1. Go to Custom CSS in Customize Journal Style.

2. Copy and paste .b-repost-item {display:none} into the Custom stylesheet box and save changes.

Voila! As you can see the boxes are hidden on the comm now :)

FYI I did not write this but it was doing the rounds on my f-list and I encourage you to share it.