Lead didn’t fill out the unsubscribe form but the unsubscribed reason states that “Customer complaint received from ISP”
There is an arrangement between an email sender and an Internet Service Providers (ISPs) called a feedback loop. When the lead marks your email as spam, this loop allows the email sender to notify the email sender (Marketo) that they have marked it as spam. Marketo automatically changes the value of the field “unsubscribed” to “True” and the unsubscribed cause as "Customer complaint received from ISP" for the lead. You can learn more about feedback loops in the below document.