For what it's worth, I can compare this scenario with using media player skins. You download skins from the internet, usually not knowing who wrote it, and even in those cases, there is no license or terms of use provided with them, just the little file you can add to your player. It is generally accepted among the public that such items can be freely modified and redistributed without violating the author's wishes, and if they don't want people to go about modifying it they can bundle a Creative Commons Attribution - No Derivatives license with the skin.
Same idea for signature codes, because nobody really cared in the past if someone modified and adapted their signature for something else, it's accepted behavior. So if someone really didn't want you to change their signature, they would write that, or tell you directly. If you're really paranoid to enforce that then license it under one of the Creative Commons licenses. But neither of these can be classified as plagiarism because the derived work does not belong to them, only the original work (the latter case, if the license is breached, is a trust violation).
Stealing the signature code and then claiming it's yours is a different story that's definitely plagiarism and also a trust violation.
So if a designer just posts a signature code out in the open, chances are they don't care whether you modify it or not. If they did they'd contact you after you use a modified signature.
The question is really whether writing that's published (whether it's in a book, news article, or an online discussion forum like bitcointalk) should be treated the same way as code that shows up on the internet in one form or another. I don't have the answer to that, and I don't know if there are any standards as far as copying code goes. My opinion is that it's not ethical to use code that someone else created without their permission, but with respect to the rules of bitcointalk I don't know if doing so would be a violation of the plagiarism rule.
All writings are implicitly copyright of the entity that wrote them, by law. If the copyright holder has legal powers to prosecute you then they're most likely going to do that if you don't include a copyright attribution with © year author etc. But if they don't, like bitcointalk users, then there's no reason to add an attribution to the stuff you copy, a citation (the person/place you got it from, without a ©) is good enough. But some people want you to contact them for permission if you're copying large parts of it, and even in those cases, it's either verbal or written permission usually they don't ask you to add a copyright attribution to the stuff you copied.
So, the answer is without a citation for a signature code, you can't accuse someone who
modified it, or copied it without stating who designed it (especially if the sig is already on the designer's portfolio like Jayce's) of plagiarism, but copying it and claiming it's your own
is plagiarism, but moderators don't seem to go after those offenses yet.
That guy ignored the designer when he tried to contact him about his copy, so a reasonable person can discern that he didn't give permission to anyone to modify his signature.