How to deal with repeated resubmissions of a bad paper (as a reviewer)? (Context: theoretical computer science.)

I just received a review request; I have previously reviewed three previous versions of the same submission (twice for conferences, once for a journal), recommending rejection each time.  For one of the conferences, there was discussion between reviewers and PC members; all reviewers agreed that the paper was “strong reject”.

Between each iteration, the paper has been lightly revised, but never addressing two fundamental problems: the novel content is very insubstantial, and the write-up contains substantial self-plagiarism.

The author(s) have/has extensive publication records, containing some genuinely significant work, but also a very large amount of repetition, often to the extent of self-plagiarism.

My current response is: accept to do the review (first letting the editor know that I have reviewed earlier versions and recommended rejection); then see what significant changes have been made, and if the major problems haven't been addressed, re-reject with essentially the same criticisms as before.

A generous referee might reasonably judge the work substantial enough for a third-tier conference, and not notice the self-plagiarism; so I guess the paper will probably strike it lucky and get published sooner or later.  Is there anything else I can or should be doing to forestall this?  Or should I accept that bad papers do get published, and that the damage the authors are causing to their reputation with referees/editors along the way is enough censure?
This type of repeated submission of junk is something that I truly hate, but also don't know any good way of dealing with.  The problem is that the same freedom that supports novel science also leaves room for this type of "publication-shopping."  In essence: any more unified method of "disapproving" junk papers, to prevent them from wasting everybody's time, will also work against highly novel papers that are meeting resistance from entrenched communities---see, for example, the decades-long fight Barbara McClintock faced in getting her work on gene regulation accepted.

We are thus left with the current and default system, in which each publication judges independently, and in which reviewers face frustrating situations like the one you describe.  My recommendation, then, is simply to judge the paper on its merits, of which it has little.  The biggest key strike is the self-plagiarism: once you've discovered that, the rest doesn't really matter, because self-plagiarism is an offense worth rejecting a paper over in and of itself.  

I would recommend informing the editor, along with informing the editor about the history of the paper.  They may then either judge whether to speedily reject the paper (and possibly initiate proceedings against the authors for the self-plagiarism), or whether to ask you to formally write this assessment as a review.