coldblossom shared this info from
geeklikewoah here There'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. (I think if you click
this it should take you there.)
2. Copy and paste .b-repost-item {display:none} into the Custom stylesheet box and save changes.
It apparently has been working for some people, so sharing the info.
Edit:
So people who know more than I have been looking at it (in the comments) and it appears this is actually not that helpful; it just gets rid of the boxes, like the Greasemonkey script, but doesn't prevent the crosspost.
But it can make it more difficult for people to get to the crosspost boxes, so there's that.