The sig banned message is passing a loud message to who ever sees it. You commit plagiarism, you get punished so I don't support the idea of removing it.
I wasn't even aware of this until I read OP's post. I
Their sig field is blank only when they will post and on all of their previous post. This message is only displayed when you click to view profile. The message will be loud and clear when you make this ban message as their sig. message on all the post they posted till now.
I was speaking about viewing their profile that's why I said "the sig banned message is passing a loud message to who ever sees it" if it was displaying on every post of theirs then everyone can see it but as it only display on the profile, (you'll have to view profile to see it that's why I said who ever sees it).
BTW: it'll be a brilliant idea if those sig banned message will be displayed on all their past, present and future post