This is a little tricky. As Marketo will not know the standard user.active field, you will need an account field, type checkbox, and have that field been mapped and sync'ed with Marketo.
But the hard point is having this field populated. As you cannot create aa trigger on users in SFDC, you will need to have a batch apex code that reviews regularly all the users that were recently marked as inactive and mark the corresponding accounts, and does the opposite when eventually the accounts have been assigned to someone else.
As a complement, you can also create a workflow on accounts that, each tile an account in updated, checked whether the user is active or not and checks/unchecks the field.