Many years ago a friend of mine had a horse that would chew anything he could get in his mouth - your clothes, with you in it, his blankets when not on him, and best of all his reins.
He was an ill mannered bargey horse and no one like to hold him whilst the owner walked the course at shows. Se could not leave him in the horsebox because he would paw and kick and he would not tie up outside without trying to kill himself.
At a show I got landed with holding him whilst the owner walked the course. As soon as I had the reins over his head he grabbed them and started to chew. I slapped him. Full strength, with the flat of my hand across his muzzle and yanked the reins back. His head went up and then he stood looking at me puzzled that he was stopped. He took the reins again and I repeated my actions. The third time he just took a rein in his lips but my reaction was the same.
He stood looking at me for several minutes and then dived his head down for grass. I pulled on one rein to get his head up but he only raised it about 2 inches before lowering it to continue eating.
Now, I could have really jerked on the rein so it hurt him but that could well bruise his mouth which I do not like so, I used my foot and gave him a short sharp kick on the front of his nose. He felt it as he was meant to but it did no damage, His head came up and after a minute he tried again. Ditto from me. He then stood still watching things going on around and about.
His owner had finished walking the course and was sat on the bonnet of her car drinking a coffee. I walked over and handed her the reins. As she took them so the horse snatched both into his mouth. He got an open handed slap from me but the best part was, he thought it was the owner who had done the punishment and he stood staring at her in shock. He never chewed his reins again
You need to get tougher with your horse. Insist on manners at all times and if you do not have control in a halter lead him in his bridle and if he will not raise his head from eating use your foot to get him to do so.