I would try a grazing rein. It's this thing that prevents a horse from putting his head down to graze while your on him. I would first do it on the lunge because some horses don't like it and rebel. I used to use it on a pony who had a nasty bucking problem when he got bored. We changed up our routine and stuck the grazing rein on him and it prevented him from getting his head low enough to do anything.
Hope this helps