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You can claim legitimate interest and keep certain information that would allow you to reidentify them for a reasonable amount of time (2 years?).

I haven't got a solution here, but it's extra friction for them to do this, you have a time window for processing and not all trolls will bother, so you would still have a better experience for other users by implementing it

You can still keep information.

Lie about it. Let them try to prove you did what they asked.

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GDPR penalties are steep, I'd highly recommend not doing this. You only need one lawyer with too much free time to test you and ruin your life.

No one has ever been ruined by GDPR.

So, you're banking on not being the first person to get caught?