It is a good strategy to try to be diplomatic even when faced with offers that are borderline offensive. It is also a good economic strategy to avoid entering into an offer-counteroffer situation if you think the first offer is way too low. (It creates an Anchor) - The anchor creates a psychological bias that can make the outcome of a negotiation end up closer to the anchor than in a non-anchored case. Thus it is smart in and by itself to dismiss a "lowball" offer without counterarguing.
While I have no direct experience with Russia - I have had some interactions with other cultures where "saving face" is an important driver of behaviour (specifically, Thailand and China). I have found that - both professionally, and personally, a "thank you very much, but no" without any arguments or justification is best. (In China, it is as I have experienced in fact rude to even say no. You would say "let us discuss it later" or simply decline (without using the word no) to discuss the matter at that point in time. You might even say "yes" with moderations to mean no! )
Any sort of justification can be construed as an argument or an attempt to haggle. It should be said in the friendliest manner possible. So my advice for Tania would be something along the lines of this:
Thank you for considering my room and making an offer on it. I am, unfortunately, not selling it [right now].
If said in person, it should be said with the same tone of voice you would have turning down a cup of tea. As-a-matter-of-fact and with a gratuitous tone. If written, it should be written as informally as available. "Hi Oleg" (I know that familiarity, nicknames and honorifics are pretty complicated in Russian, so i'll leave it too you to decide what is familiar enough...)
You may, if courtesy allows it, include the ending [for 200.000 $] instead of [right now]. That will be a bit more leaning on him to make a new offer - but it is also clearer in your lack of enthusiasm of the offer itself. Or you could omit the [right now] for nothing, but that could make it more final, that she is indeed never interested in selling.