Hacker pirating on our wifi?

I have been receiving breach of copyright letters from my ISP, saying that there has been breaches of copyright on our wifi, listing shared/downloaded movies like Justice League, Kong Skull Island, etc. I am pretty sure no family members are doing anything, and we don't have people around often, so I'm... show more I have been receiving breach of copyright letters from my ISP, saying that there has been breaches of copyright on our wifi, listing shared/downloaded movies like Justice League, Kong Skull Island, etc.

I am pretty sure no family members are doing anything, and we don't have people around often, so I'm really not sure what the cause is.

We've changed the WiFi password in between but this does not seem to stop it. Though ISP has given an IP address from which the files have been shared/downloaded.

This IP has not changed for the months that this has been happening, and I have found an address through this IP, but I am not sure if it is genuine or spoofed/etc.

Question is; what is the best way to deal with this?
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