Marketo has a built in global unsubscribe functionality that will automatically prevent anyone who is opted out from receiving non-operational emails. That said, are a couple reasons why someone may still be getting emails, even if they're "unsubscribed"
1. The lead is unsubscribed from a custom field, like a newsletter subscription field, but not the primary opt out field. The system-controlled opt out field is called "Unsubscribed" in your Marketo instance, and it has two values: True or False.
2. The lead received an "operational" email, which has overriden the global unsubscribe settings. This feature can be activated for any email, but is really only meant to be used in instances where the email cannot be considered spam. You'd use this only in situations where the person is expecting an email, or needs critical info from the email, such as for an event registration or an autoresponder, or in certain instances, a critical customer communication.
Hope this helps.