Belgian here. I don’t use "ploum ploum" either (else I would never have this pseudo) but realized over the years that a number of people use this expression. (look on twitter for "ploum ploum" for example).

And, yes, lot of spam is because this email was in multiple exposed account but I’ve received lot and lot of "fresh subscription/confirm your emails" where it was obvious people were simply writing whatever in a field.