There is an implied social relationship here. They are asking for a considerable amount of work, so they should have the courtesy to respond. Something as simpl

There is an implied social relationship here. They are asking for a considerable amount of work, so they should have the courtesy to respond. Something as simple as saying that they received and considered the work but they are not moving forward with the application. If they cannot say anything further, they can make it clear that company policy does not allow them to comment on applications so no further communications will be reviewed.