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Depends on the front-end you use (peakd, hive.blog, ecency, ...)

I almost always use Peakd. Sorry if it's a stupid question!

There are no stupid questions.

Click on the 3 dots under a post/comment then select downvote.

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You will then see a slider that allows you to choose how "hard" you want your downvote to be

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Finally, click the downvote button to confirm.

Thanks :) And this will help to mute our scammers?

You might need to raise your reputation to fully mute people with a higher reputation than you, but it will clearly show the path to others.

Would a dolphin account do it? I can ask a friend to downvote them too next time...

It's not about how much HP you have but your reputation.
Unfortunately, the reputation system is a bit messy, making it sometimes difficult to counter scammers who have hacked an account with a high reputation.