Spammers usually want to tell you that their message is not spam.

But even if they admit that a message is sent without solicitation, they try to tell you that there's no point in doing something against them. That's why they then say: "This is a one-time mailing."

Usually, it of course is not. But since this phrase is used almost exclusively by spammers it makes for a nice filter.

Your anti-spam filters could look for "one-time mailing", "one-time-mailing", "onetime mailing" and other variations in the body of incoming messages and place matching messages in the "Possible Spam" folder.