The only way to do this is to export a report of inactive users from SFDC and copy their email address into a Lead Owner Email Address filter in a smart list.
The only way to do this is to export a report of inactive users from SFDC and copy their email address into a Lead Owner Email Address filter in a smart list.
I think Elliott's solution is the way to go. Once a user is inactive in SFDC it is impossible for Marketo to see those records, even if you reactivate them in SFDC.