As harsh as it sounds, you need to sit out there, and identify the instigator cannibal and get rid of/cull/eat whatever the hen. If you are lucky, you just have one instigator.
I agree you need a feeder that has food available at all times of the day that they are awake. If they are 6mo old, get them off of the baby food and onto layer food. Chickens if given adequate space and enough nutritious food almost never go cannibal. This is a problem of overcrowded, stressed and/or nutritionally deficient birds.
Chickens eat a little bit every few minutes, they do not gorge except right before they go to roost, or if you offer something extra tasty.
Do not hope to cure her/them, it is a horrible habit, and given the opportunity- she/they will do it again. They pick at the softest, least protected part (the vent) and eat the unfortunate bird alive. The makings of a horror movie.
Honestly, unless you really are attached to these birds- I would just cull and start over with a bigger and better set- up. Or likely you will keep losing birds until you have one happy cannibal left- who will try to do this to whomever you try to introduce.