It's because they post a huge wall of text, and if you try to scroll up, it skips to the text right above the first line of their post.
Personally, I've found that reducing the font size in Settings makes it possible to view their entire post, and thus their name. Right-click the name and ignore, then return the font size back to normal and report their account name (that's more important than their character name anyway). It's a cumbersome workaround, but it has worked for me.
Yes, set your font size to 10/11 in Options -> Social and then drag your chat box to maximum size. That should show you the names and allow you to report/ignore them.