Someone/something is making thousands of impressions per day, is it harmful?

Someone is purposefully doing thousands of ad impressions in my game, I don't understand his motivation though. Is it some kind of harmful action?

Usually with my DAU, users watch around 150-300 ads per day, but for the past two days something or someone is making thousands of impressions.

I wonder if I can take any actions and even if I should? What kind of harm does it make to my app, or maybe my revenue? It does lowers my average eCPM though, because he doesnt install anything, just makes a lot of impressions.

I think some advertisers might ban my app id because of that. But what really can I do? How to stop him?
He is watching banner or interstitials, not rewarded. Even though I have a hardcoded limit to 1 ad per 90 seconds, he is somehow making more impressions.