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>Consider physical junk mail...

Where I live it takes a sticker 'no spam' glued to the mail box and no more junk happens

I've lived in several places where this practice is common and none where it remotely works - it's totally ignored. Where are you based?

I tend not to leave any specific info about myself online, so the answer would be pretty broad - mainland Europe

I will just second that this works in my country (Lithuania).