How does one approach reverse-plagiarism -- being attributed authorship without contribution or consent?
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...