Foodpump, while such a letter seems like a good idea it can backfire fast, as unethical bloggers will use it to pan your restaurant without even visiting it.
You're better off declining on a one-to-one basis. That is, if they call, politely explain that your policy is not to comp, because there are far too many bloggers in the area, and you can't afford to host them all. Basically the same message as your proposed letter.
Pete is absolutely right: legitimate reviewers not only do not identify themselves, they take great pains to hide who they are. In some cases this can make for funny stories. See, for instance, "Born Round," Frank Bruni's autobiography. Frank is a former restaurant reviewer for the NYTimes.
You've identified the problem yourself: The vultures react to hearing about you from media that did not look for freebies or even tell you ahead of time that they were visiting.
As an aside, while I recognize they have reviewed you based on a single visit for economic reasons, I don't like that approach. What if you were having a bad night? Or the reviewer didn't like the particular dish (s)he ordered? The standard approach is to make a minimum of three visits. Sometimes, on the third visit, the reviewer identifies him/herself after the meal, so as to arrange a kitchen tour. But I digress.
Whether to comp or not is a business decision you have to make. FWIW, overall, comping out of town travel writers is more beneficial than comping in-town food bloggers. In such cases, more times than not, the Vancouver travel bureau will be your initial contact on that, so you'll know it's a legitimate request.
That's nice, does his mother write the blog too?
Putting aside the comp issue, I would always expect a reviewer to have at least one guest. There are several reasons for this, which we can discuss if you like.
.....there are over 150 food related blogs.....
Nothing to do with the issue, but I can't help playing editor, and this is one of my pet peeves. If you do produce that letter, there are more than 150 bloggers, not over 150.
Sorry! 