Have seen this in a bunch of places but wanted my f-list to have it, as well.
The following line of code, though, doesn't just hide the repost boxes to you like the Greasemonkey code for Firefox going around does. It completely removes the option for everyone viewing your journal or community.
1. Go to Journal > Journal Style
2. Go to Customize your theme
3. Go to Custom CSS
4. Copy and paste .b-repost-item {display:none} (make sure you're just getting the bolded text) into the Custom stylesheet
5. Save changes
Now not only can you not crosspost comments, but anyone viewing your account can't either. This doesn't get rid of the reposting option when you're making a new entry, but at least that one's easy to avoid.