gastro guard, but it's expensive, and I wouldn't use it unless your horse has severe ulcers because of the expense. another course is ranitidine (hopefully I spelled that right!), which usually comes in a pill or powder you add to their feed for several weeks and then as needed.
the best thing though once you've got it under control is feeding a lot of grass hay. horses aren't meant to eat 2x a day, so the stomach acid builds up and ulcers form. so feeding them multiple small meals per day will really go a long way.
there are also some herbals you can feed that help, such as aloe vera juice, slippery elm bark, peppermint, fennel seed, and a few others I can't think of.
do not feed anything w/ devils claw, and limit use of bute and banamine since those will irritate it.