Seems like only admins and global moderators can change post owners right now.
So if so, you might be able to choose these settings on the permissions screen?
Or even if the button does not show to anyone who does not have this permission.
Several days ago, we had one spam user, that joined to our forum and wrote the same message as reply to more than 100 different topics...
We caught it 1 hour later, and deleted its account and posts... However, some of the categories shows "no new posts" on the right side, although there are clearly posts before this spam message. I would assume the post before the spam post should be shown.
I suspect this is a bug, so FYI. Here are pics from our forum: