There are two major reasons horses become overweight: they are fed too much feed or they have a genetic hormonal problem called equine metabolic syndrome that contributes to weight gain on a normal ration. Regardless of the cause, the concern is that these horses will develop laminitis and founder. To prevent laminitis and decrease weight, a specific feed program must be used. In most cases, grain cannot be fed to these horses at all. There is a commercial feed that can probably be used in these horses but regular grain cannot be fed. Also, these horses cannot be kept on pasture because of the high starch in grass and the inability to control the amount of feed.
Most of these horses should be on a hay-only diet and the hay should be tested to make sure it has less than 10% nonstructural carbohydrates. Because a hay-only diet is low in vitamins and minerals, it is a good idea to use a vitamin and mineral supplement to help balance the ration. I know these recommendations are difficult as I get a lot of resistance when telling clients that their horses cannot have grain or pasture. However, the alternative is laminitis and founder that is extremely painful and can lead to the horse's death. Stalling horses is also not a good idea long term so I believe the best option is to choose a small area of pasture and fence it off from the rest of the pasture. You can keep this area plowed and void of grass and yet allow the horse to have some exercise. This may not be required forever but certainly until weight loss occurs. Some horses will never be able to graze in the pasture again.