I recently received a request to referee a manuscript for a journal and, while doing so, I pointed out a crucial mistake undermining most of the calculations. After two unsuccessful answers on the part of the authors (where they addressed other concerns of mine, but basically ignored the mistake I was pointing out), I recommended the journal reject the paper and they did so. 

Very recently, and within a couple of weeks after this rejection, the authors have openly posted their article on arXiv, without correcting their mistake. While I understand that manuscripts from arXiv should be taken with a grain of salt, as it is not peer reviewed, making a clearly incorrect result public seems wrong to me, both from an ethical standpoint and from a practical one as it might mislead researchers.

I am unsure how to proceed. Should I ignore it or take some measures such as publishing a reply/correction? If the latter, is there any way to do it for arXiv submissions? Is it possible to do it anonymously?