Personally I think it would be best for you to leave a negative review.

I had a negative experience recently, with a professional teacher who insisted on discussing a topic even after I explicitly said "I don't want to talk about this," and who insisted on doing nothing but conversation practice in spite of the fact that I had asked specifically for help with grammar and pronunciation (and I could barely follow what she was saying, so I learned practically nothing). Unfortunately, I had made the mistake of paying for a package of several lessons with her, because we'd had another lesson previously and I'd thought it had gone well. I didn't want to leave her a negative review, because I wanted to request a cancellation of the package, and I was afraid that if I left a negative review, she might refuse -- and then I'd either lose the money or have to take more lessons with her. So I left a positive review. She was at least nice enough to let me cancel the package.

I still sort of regret that I didn't leave a negative review, though, because she absolutely did not listen to anything I was asking for, never checked with me to even see if I was understanding what she was saying, and talked by herself for most of the lesson time about a topic I had asked her to stop talking about. :/

As Brett says, a better way to find out whether a teacher is good is to check the ratio of students to lessons. If the teacher has had 50 students and has only given 60 lessons, that means almost none of those students went back to the same teacher again, which is not a good sign. On the other hand, if the teacher has only had 10 students but has taught 60 lessons, that means quite a few of those students have gone back to the same teacher several times. That teacher is probably pretty good.

Sorry to read about your experience! I actually contacted italki support a couple of years back about this exact issue, and their (unofficial) advice was to look at the ratio of lessons-per-unique-student to get a better feel for a teacher's quality. I've also never seen anyone below 4.9/5.0, but I've found that if you target teachers with, say, 8+ lessons-per-student, you're probably on to someone with a good track record.

I’ve begged the folks at italki to make the ratio of repeat customer classes (not including the first lesson, which is *not* a repeat) more visible, but all they do is make the statistics harder to find. You used to be able to see the number of students (not just lessons) on the search page, but now we have to go to each profile, find the stats, and do the math. Another problem (that I haven’t bothered to tell them about, not that they ever listen) is that 1/2 hour or 45 minute lessons count the same as a full hour. Now, obviously teaching 100 hours is not the same as teaching 100 half-hours. Not only is it half the experience, but it’s a lot less challenging to keep a student’s attention for a mere 30 minutes, plus the cost is lower, as are student expectations.

One more thing — since the site “update”, it seems to me that the profile page might not be showing all the written reviews. Am I wrong, or is there some sort of selectivity going on here? Also, you used to be able to click on students’ reviews to visit their profiles — a great way to see how they reviewed different teachers. If a student writes glowing reviews for ten teachers and then writes faint praise for the eleventh one, it’s easy to interpret that. 

That sucks! Sorry you had to experience that... I know there was a guy who did a review on YouTube saying he has had 3 out of 15 teachers who weren't the best, I'm not sure how he reviewed them on here tho, i guess you just have to shop around till you get a teacher that is good for you... atleast there Is a trial lesson option and it's not full pay just in case you end up in another situation like that

(@Phil - I actually just yesterday booked a lesson with a teacher whom I found through your profile page, and chose based on the fact that you had taken eight classes with her. While browsing, I noticed that some of the teachers in the teacher list on your profile page did not have any reviews from you at all. I'm not sure if that's because some reviews aren't displayed, as you suggest may be the case, or whether it's because you may not have taken classes with all the teachers who are listed on your profile page, or because you may not have left reviews for all of them. In any case, it would certainly be nice if there were a better system for displaying teachers' statistics on the site.)