How does one approach reverse-plagiarism -- being attributed authorship without contribution or consent? For reasons I am not going to delve into, I was given second authorship outside my consent on a couple of conference papers I have absolutely no contributions on.
To resolve this, I have been repeatedly going back-and-forth with ACM to remove me from those, which keeps ending up with the head author very strongly insisting I did contribute, and ACM responding with the written equivalent of shrugging and walking away.
This whole situation is very bizarre and I am sincerely at a loss as to how to even begin approaching this. Plagiarism is complicated enough despite it basically being the most studied academic subject of all times. But how does one even begin to approach it happening in reverse?
The head author’s insistence that you contributed is irrelevant. One simply cannot make someone a coauthor without their consent. I suggest that you reply to the chain of emails with something along the following lines:

Dear [ACM editors],
Thank you for your assistance with the issue of the coauthorship of [paper]. My colleague [head author], cc’ed, is being too kind when he says I contributed to the project. Perhaps there is some truth to that; regardless, for personal reasons I do not wish to be made a coauthor, and do not give my consent to be listed as a coauthor of the paper. I make this decision without implying any opinion as to the content of the paper, its quality, or the question of how much I contributed.
Please confirm that you are removing my name from the list of coauthors [optionally, you can add for added pressure if you expect more resistance:] or I will be forced to escalate the matter to the [Editor-in-Chief/Advisory Board/etc].
[head author], thanks for the lively discussion and for your generous suggestion to make me a coauthor. If you wish to mention me in the Acknowledgements section that is totally fine.
Regards,
Layman

Once you expressly state your refusal to be made a coauthor in this way, it’s difficult to imagine them “shrugging and walking away”. My guess is you simply haven’t been assertive enough in your earlier request. Good luck, and do update your question to let us know how things turn out with this curious affair...