If I get continuously get emails sent to me that I did not request and I can't unsubscribe to, then it's spam. Maybe companies should make sure they're not sending emails to the wrong person, because I'm just going to keep marking it as spam when it comes my way.

Exactly. "Poisoning" the spam filter is nothing more than sending an alert back to upstream to deal with it when and where it starts to hurt business. Typically it's the only feedback mechanism taken seriously and thus a service to the community.

But...it is spam? If they don't give the tools necessary to stop the spam (unsubscribe or a link to "received this by mistake") then it's spam - intentional or not.