@Yulia107
I agree that a review that simply says "b" should be removed on the basis of being irrelevant. Certainly I would persist in such a case. Clearly it is a retaliatory review as, apart from what you have explained about the communication, your other guests always make a point of stressing how clean the place is. Unfortunately, Airbnb do not seem at all interested in removing retaliatory reviews, so I would focus on the irrelevancy clause if you are going to pursue this with them. I don't know how any sensible person can argue that "b" is a relevant review!
Unfortunately, guests don't always read things properly. Although your listing titles say 'private room', they also say 'apartment'. Just to make it more obvious, maybe specific private room in shared apartment, or remove the apartment bit from the title.
You do mention under things to note that the listing is a private room, but I am afraid guests often don't read this far, so I would put that near the top of the description as you describe the apartment in quite a lot of detail before the mention of private room/host being on site.
I know it should be obvious, but I learnt early on in hosting that what is obvious to you isn't always so to others.
Also, I would suggest you write responses to reviews a bit differently. Firstly, if you only respond to negative reviews, you make those stand out on your profile/the listing and really draw attention to them. Respond by all means, but then also make a point of leaving a nice response to all the guests that leave positive reviews.
Remember that your response is not there for the benefit of other hosts (you tell other hosts to beware of these guests). The response doesn't appear on the guest's profile but on yours/your listing and so should be targeted at future guests. It is therefore not a great idea to be aggressive here as it's very off putting to potential guests and I would certainly not write that you are going to hunt this person down! It is maybe not a good idea to write negative comments about locals. If you don't want to host locals, then don't accept them.
Keep your responses factual and unemotional and respond only to what the guest has written in the public review. There was actually no reason to even respond to the "b" review. The future guests cannot see the ratings or feedback these guests left you, so really no reason to bring it up.