I'm guessing that some spammer detected you on line as knoppix@ youraddress.net & added you to his list of functional addresses. Short of using a firewall, not much you could have done to prevent it. This is not a Knoppix problem , just an internet user problem.
I had a similar, someone started sending me spam in Russian, I think they got the address from me contacting a Linux message board. I traced the source to Montevideo but that was all I could do. I reported it to my ISP, asked them to handle it & it stopped.