How can i stop my 2 nieces having a baby soother/pacifier?

My nieces who are now 4 and 6 years old are still having their baby soothers/pacifiers. We try to make it stop by getting rid of all the pacifiers in the house and everything but they always throw a tantrum and we always have to end up giving it to them. They both should have stopped when they were 3, but we don't know what to do, any suggestions?