We're actually going through some clean up ourselves to get our HR license count down.

Here's the information we were given on how HR licenses are calculated:

HR license counts are not derived from roles like other subscriptions are, they are derived from the following query:

• This daily measurement is run on the [sn_hr_core_profile] table using the following criteria: 
  • User.Active is true
  • Employment start date at or before Today
  • OR .Employment start date is empty
  • Employment end date at or after Today
  • OR .Employment end date is empty

In our instance, we don't generate HR profiles for our Contingent Workers (Contractors), unless they are a people leader and have direct reports.   So your suggestion to enter an Employment End Date in the HR profile for those users that are contractors, that would remove them from the license count.  Just make sure you don't have any integrations that would blank that field out in the future.