It'd be up to a judge and/or jury to decide. If one can establish that the intent was to ignore one's attempts to unsubscribe, it'd be a pretty clear-cut violation. Most reputable senders have an "opt out of all further communications" checkbox (with some fine print about legally required and transactional emails). Pretty much the only way to bring a private action under CAN-SPAM though is to be an ISP and show "actual damages".