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Oleh, I think it would be safe for you to try to eliminate ranking for that keyword. With that low CTR, If you don't do it then Google will do it for you, which would be worse. It is my understanding that CTR is a signal, but that if someone is manipulating it they would have to keep doing it for the long run in order for it to stay effective. It wouldn't be a permanent thing, as you would slowly rise up again when the attack stopped. You wouldn't be the first or the last to block all traffic from the Philippines. I'd start there. That said, I have no idea what the title and description are in the SERPs, or what the landing page is, or if the content is relevant to that query. All of these things matter. Also, I have doubts that CTR for a single page or query by itself would cause a 10% drop in traffic across the site, even if the global CTR did lower because of it. As Roman suggested, have a very close look at your link profile using a tool like Link Detox or Moz Link Explorer, just in case they are hitting you there as well.