If needed you can manually close you’re open bills and/or POs if you will no longer be tracking an outstanding balance. Both of these options can be found in the drop-down menu near the top-right of the bill or PO.
Bills:
POs:
Closing it is telling Knowify that you no longer plan to track the outstanding balance or any payments on it, where deleting it will wipe it from the system entirely, removing the cost allocated to the job(s).