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Just as EGOL describe it. If you're on Amazon AWS then you can use their CloudFront as CDN. But also you can observe source of traffic. Could coming from one country, one IP range or one user-agent. There should be some kind of pattern and you should investigate it. Then just need to make rule to block that traffic or just redirect them to one static "hello world" page. I was also victim of such traffic, but was from humans trying to depleting an AdWords daily budget. Once budget it over ads was stopped showing, after few hours they recalculate clicks, some funds was returned, ads are shown again, they click it, budget is over... and so on.