Agree with the others. You need to be tough and stop the treats altogether and scratch is one of their favourite treats. As the others have said they will eat it first and then not eat their proper ration. If they are free ranging then they don't need the veggie treats. Meat scraps are actually beneficial....chickens are omnivores like us and like meat and fish as well as insects, grains and vegetation. Part of the problem at the moment is that there is not enough protein in the scratch compared to their crumbles and with the extra veggies their overall diet will be low in protein unless you are feeding huge amounts of meal worms. Of course, they will be very used to their current regime and may go on hunger strike for a couple of days when they don't get their scratch, but stick it out because the scratch is not good for them.....Last year I butchered my neighbours hens for him as they were very poor layers and he wanted rid of them. They had been fed about 50%scratch and when I plucked them and opened them up they were obese and their organs encased in solid yellow fatty deposits. It was no wonder they were not laying well. It taught me a thing or two about diet and I'm much more strict with my own hens now and how much scratch they get.  
 
Also, throwing the crumbles on the ground is just a waste and will encourage wild birds and rats, both of which can bring disease. I occasionally throw pellets down when my flock are confined, just to give them something to do to help prevent boredom, but when they are free ranging, it is best to have a feeder hanging in the coop or run so that they have access to pellets/crumble throughout the day. They will come and go to get food when they want it and then head back off to look for more interesting things to do.