Thanks everyone!
I'm wondering if when you're running reports excluding the Marketo User, if this is just reports of Activites, or are you able to exclude from the "Last Activity" function, too? Basically, we want to re-assign Accounts that have not been worked by the rep in X time, which is the use case for "Last Activity" specifically, however when Marketo is logging Activites, this field is no longer an accurate depection of whether or not an Account is being actively worked by a rep. So maybe this is really a Salesforce question...