This is an actual law you may contact your local animal control to handle the problem if you want or you can ask them about the law as well. As much as I would like to trap them like you do if you get caught with releasing them without an actual permit...
Hi I'm sorry but mallard ducks are protected by the U.S Fish and Wildlife. Unless you have a permit you may capture or handle them. This is a page from their website that says stuff about them
https://www.fws.gov/cno/conservation/MigratoryBirds/pdf-files/Ducks-in-pool-4-27-18.pdf