Quick Answer:  To stop a horse eating too much grass:
1.  Best option is to create a grassless track, they move and can't overeat
2.  Use a soft, strong grazing muzzle preferably with a grazing slot to allow a normal bite
3.  Create a dry lot for part, most of the day

Why should you stop a horse eating too much grass?  Well an overweight horse is at a big risk of laminitis, this is horrible and painful, you must be aware.

Using a good solid but soft grazing muzzle solves a lot of problems for a horse owner who sees their horse putting on weight.  The amount of grass a horse can eat is often too much for their system to cope with, modern grassland is much more rich than they are bred for and often they don't even need to move around to get it. Wild horses walk for miles to find what sparse grass they can, our overfed ponies have it on a plate.

Sugar is a big culprit for laminitis and extra weight so not only will you have to reduce the amount they eat in grass restriction, (see ways above to see how to restrict) but any hay you give them to top up fibre will also contain sugar and starch and will likely have to be soaked in order to reduce both.

Getting laminitis is not only a very difficult time for you and your horse, but they remain at higher risk of a relapse for ever. 

https://www.horsemuzzle.com/blogs/grazing-muzzle-advice/how-to-stop-a-horse-eating-too-much-grass