Cattle producers using nose flaps on calves at weaning say they are the least stressful way to wean calves. Inserted in the nostrils the flap hangs down over nose and mouth preventing the calf from nursing. Flaps do not restrict calves from eating grass or drinking water. The flaps are reusable as well, spreading the costs over multiple calf crops.
Source: Angus Journal, February 2022, page 110. Link.
Also see: Weaning basics keep calves healthy, AHD, July 23, 2019. Link. Heather Smith Thomas covers nose flaps and fence-line weaning.