I have been dealing with the same problem for two years with my stallion. He is pretty good if I keep driving him along the path, but if I relax at all or quit driving with my body, or stop paying attention, he tries to grab anything within reach (grass, leaves, sticker bushes, ANYTHING). It drives me nuts. Right now I am just waiting for the snow to cover everything so I get some peace :) When he does it I drive him faster or in a circle or two, but it doesn't really discourage him.
I keep hoping he will eventually get past it. Sorry I have no real advice, but I share your pain :).