While I understand what the Cafe in this story is worried about, trying to pre-censor customers is not the best way to proceed:

What I was told, in a nutshell, is that the café staff has encountered a stream of would-be critics “with attitude,” predisposed to take issue with or be critical of the business. Whether or not this is a correct perception, there are many more outlets (Yelp being only one) for customers and consumers to voice opinions about businesses on the Internet. And there’s little most of these businesses can do about it, for better or for worse.

For good or ill, you just have to take the commentary. Anyone can be a critic now; it's no longer limited to the food guy at the local paper. Are you going to get unfair, negative, personal attacks? Yes, you will - but putting up a sign that asks people not to do that isn't part of the answer, IMHO.

Here's what I do, in my role as product evangelist: I have search feeds set up for various terms that might come up in a discussion of our product: Smaltalk, Cincom, Cincom+Smalltalk, etc. I scan the results every day, and I respond to the negative ones - not with exasperation, but with questions for details on the problem tat generated the negative post. Is that going to solve every issue? No - there are trolls, and there's nothing you can do about them. Over the long haul, your dedicated customers will recognize and tune out the trolls though.

Ultimately, you can't stand back and to stop the tide of commentary. The best you can do is jump in and try to ride along.

Hat tip Mathew Ingram, which is where I noticed this.