What are the most effective treatments for flat warts on the face?  

Facial flat warts are notoriously difficult to treat. I’ve had success using Tretinoin 0.025-0.05% cr HS MWF HS and iniquimod 5% cr sparingly on the lesions only HS T and Th. It is often slow but steady progress without scarring as long as the patient does not have significant actinic damage.

Agree with the above thoughts. I usually do gentle curettage over several visits. Or really light electrodessication and gauze "curettage". Same as DPNs.