When a prospect unsubscribes or marks an email blast as spam, NationBuilder appropriately opts them out -- but their status remains as 'Prospect' rather than switching to 'Non-supporter.' Based on the documentation How support status is determined, I would expect this unsubscribe action to change their status to 'Non-supporter' since they have clearly opted out.
1/20/17: The How Support Status is Determined HOWTO has been updated to clarify that negative actions will make supporters become non-supporters and does not affect the prospect status. "Prospect" as a support status is generally only relevant when email blasting people. When someone unsubscribes from a prospect email blast, she will no longer receive emails sent to your prospect list. Further, we recommend having a stringent process emailing prospects and using their engagement / non-engagement in that email to manually change support level. --C.J. MinsterCheng
1/3/17: Thanks Lindsey! Definitely something we should look into changing - I'll also talk with our writing team about updating this page if necessary.