Of course like everything else, some agencies practice this more than others.  My personal POV on the subject is "if the experience was sub-par, why do you want to use that agency again in the first place?" but I know that was not your question, so I will give you a couple of ideas.

 
The first of course is to write your reviews under an alias, or mix it up between writing your reviews under your alias and your real handle so if the agency follows every review (and many do) they won't start seeing a pattern of who you really are. Secondly, leave out any details that would identify you to the agency, dates, times etc and you shouldn't write those reviews immediately after seeing the lady, let a month or so go by before actually submitting your reviews.

 
I still maintain that you should NOT continue to patronize agencies with such predatory practices, but that of course is your own decision to make.

Depending on what happened, your first recourse is to discuss the matter with the agency. Preferably, immediately after the session, not a day or a week later. E.g., "your ads say GFE but she wouldn't even ..." or "your ads say $$$ but she demanded more to continue the service" or whatever might be the matter. You cannot complain about "chemistry" or "she smells like garlic" or "too much spit and slobber in her BBBJ" - well, you can try  .  The agency does not have anything to do with that other than to provide feedback to the hostess, if they think it's helpful or relevant.
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You shouldn't bash the agency until you talk to them. A good agency will make amends.  
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If they tell you to fuck off, then you aren't going to be using them again anyway, so post your review to let others know about their poor service.  
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If the matter is entirely YMMV and personal and you just don't want to give a 10 for a 7 performance, that's a different matter.  

For at least one San Diego agency, it's policy stated clearly up front:  "We'll refuse you in the future if you give a review lower than 8".  Luckily their girls were 9.5 for me, so I got to write the reviews I wanted.

I guess this is why it's a good thing the editors deep-sixed my review savaging a provider who was *seriously* flakey and inconsiderate during the scheduling process.

And yes, I'm new to using Internet providers, though I've been thinking about it for quite a while, hence my presence her for some time.

Negative Reviews

Yup, posted about this on the general discussion forum.  Apparently even mediocre reviews can have an outsize impact.  Providers and agencies do talk to each other and will spread the word about anyone that writes bad reviews.  This hits both the ladies and the agencies in the pocketbook, a very sensitive spot for all of us.  I know this for a fact because I wrote a mediocre review (a 7-6) and a friend showed me the email they sent warning that I write "bad reviews".    As for anonymity, all that's needed is your handle on TER and that's easy to figure out by checking the reviews. Fortunately I was able to talk it over with them and we are all friends again (I think).  FWIW