I have 12 adult angels that love to plant nibble and I almost have gotten them to stop by feeding a few cooked, deshelled, slightly smushed peas with their daily meal. Then I cut a coupe 1/4" slices of zucchini, nuke the about 8-15 seconds, hold them under cold water to stop the cooking, then hang them on veggie clips in the tank all day. All the fish but especially the angels love having alot of veggies in their daily diet. You could try supplimenting their diet like this and see if it cuts down on plant eating. I also do this but use all kinds of fruits and veggies in my fancy goldfish tanks which are planted to help keep plant eating down to a minimum.