Proof refutation: Amateur reviews of ambitious CoRR papers
If you make an arXiv trackback you will not be ignored, in the sense that future readers of the ambitious arXiv paper may check the trackbacks.

You even get a mild form of peer review for your posts, since they write:


  Because of widespread Trackback spam we have a semi-automated editorial process that approves trackbacks for display. 


As for where to post your review, Recent trackbacks currently returns a mix of blogs, magazines, and news sites.