Hi @josh_at_MITTR,
You can do this through marketing emails, under the recipients tab is the option of "spam reports" which will indicate which recipients marked the email as spam.
While this is not associated with a contact property, you can create a contact property for this purpose. Additionally, if you have access to workflows, you can create a workflow based on the contact having marked a specific email as spam, which could assist in setting a custom contact property around spam.
Thank you,
Jenny
