It is the “Match This Property” tick box on the Details tab of a property record that determines whether a property gets matched. So if you have marked a property as ‘Sold STC’ but have left it with “Match This Property” ticked, it will still match. You can un-tick this box on the Details tab of a property’s record, or when you make the status changes through your offer - if progressing your offers correctly, the system will ask you whether you would like to continue matching the property or not when you mark the offer as accepted.
This is the box on the Details tab of the property record:
For information on making the status change as you progress your offer, see the Accepting an Offer section.