There's more to using of AI in generating post, why then can't a user of such never indicated the source of their contents to be AI generated, if you're giving out an idea, then let it be strictly on your personal own point of view, if you're collecting other ideas from different sources and yet claiming they are yours without you indicating their sources means you're not to be trusted with whatever you offer since they are not generic from you.
First, before we look into why can't a user indicate that his or her content is Ai generated, we will have to look at why some users will want to use a CHATGPT in the first place.
Mostly, those who use it, use it to find an answer to a (maybe) complicated question which they believe could be of good knowledge or act as a good information to other forum users, who might in turn want to appreciate the user by giving merit.
So considering the above, we know that those who use CHATGPT, just as the op have said, use it to make them appear more intelligent and knowledgeable than they original are, and this they do, all for nothing but to earn more merit in the forum, so if earning merits be the goal, indicating the real source of the information they are sharing might just be a goal damager for them, since the real essence of using Ai in the first place, which is to appear more intelligent than they truly are, is completely defeated.
Going back to the op, I think since the forum already have a rule against plagiarism, same rule can apply to those who use CHATGPT to make post, or better still, let the forum allow campaign managers to discipline those users involved in such an act.