This issue has been brought up before in forum #76931, and while it was agreed upon that it's an undesirable practice that warrants warnings, a clear procedure for dealing with offenders hasn't been established aside from some references to giving neutral records.
While a mail message or a neutral record should suffice in getting their attention about the problem (and I believe some Janitors already routinely warn new users about their practices), it doesn't bring to attention said user's practices to people other than the uploader and the person who sent them mail.
If the person who warned the uploader did not correct said uploader's failed tagging (due to lack of time or lack of series tagging knowledge), and the uploader does not retroactively fix his prior uploads (or worse yet, ignores the warning altogether or goes inactive), these images essentially become "unfindable", for tag gardeners and regular users alike. For this reason, I believe that a "user reporting thread" of sorts should be made for users to report people who do not properly tag their uploads even after a few days, not so they can be ostracized by the community, but so tag gardeners can be made aware of the existence of these poorly-tagged images and hopefully remedy them.
For instance, someone who notices poor tagging behavior could make a post and explain the tagging practices of the involved users briefly (since there are varying degrees of severity):
http://danbooru.donmai.us/user/show/136354 - Uploads with nothing except chartag/copytag and occasionally a tagme.
http://danbooru.donmai.us/user/show/342732 - Does not tag his uploads beyond a bare minimum and incomplete set of gentags and a copytag, no characters are tagged even for well-known series.
http://danbooru.donmai.us/user/show/359759 - Submits one gentag/copytag/chartag and leaves the post as-is without even a tagme.
This does not mean that users mentioned in the thread get a free pass for not tagging their uploads (lest more image upload machines be created like http://danbooru.donmai.us/user/show?name=Herrmobel or http://danbooru.donmai.us/user/show?name=UnChocolate who upload tens of thousands of images with minimalistic tagging). However, it would mitigate the issue somewhat by giving these posts more exposure to people capable and willing of tagging, and gardeners are likely more willing to fix the uploads of a single user as opposed to everything in tagme or gentags:<5.
Any thoughts on this? Or is it a hopeless cause?
Updated by Serlo