<sigh>, I need to modify what I said, since I it isn't totally what I meant
I did not mean to keep unlimitied amounts of hay in front of the easy keepers constantly.
As Super Nova commented, they need a limited amount but when that limited amount is put in a slow feeder they can "keep grazing" longer because it takes them longer to get the hay. That gives those ever-flowing acidic digestive juices something to process
My EMS horse has turned into a hard keeper and has hind gut ulcers. He gets as much hay as he wants and invariably has plenty left over by morning. Sometimes he is quietly munching when I walk in the barn, in the early AM, sometimes he's at his stall door waiting for his ulcer meds.
My insulin resistant horse rarely takes a break from eating. He gets 10 - 12 lbs nightly in a slow feeder net. By early morning he generally has a few scraps left and is nickering "feed me" when I walk in the barn. I have managed to get enough weight off him that I can finally see his ribs when he stands a certain way.