The reason that they are chewing everything is that they are probable bored. When they are outside or in their stall just have something to play with. I know for my horse he like a jolly ball in his stall and one outside. Also I use a chili pepper solution to keep them from chewing. This solution is just simple water or mineral oil with chili pepper seasoning, then just rub it on everything they are chewing on. Also maybe a cribbing collar would also work.