There is an option in the system to automatically archive all "Time off" for an employee once they become a leaver.
This will need to be set up by someone who is an Admin user within Natural HR.
They will need to go to
Admin > Company > Settings > Time off Settings
In general settings you will see an option to auto achieve leaver time off and training. For this to automatically happen when a leaver is process be sure to set this to Yes.
Please note: This ONLY applies to employee whilst they are being archived and it will not archive any data for prior leavers