We used to have a pony that had this issue. She constantly would try to get the bit shank into her mouth, and the reins to follow. With her, we used an "out of the box" method to get her to stop. She HATED having her ears touched while riding. So every single time she would start to play with the bit, whoever was riding her would immediately reach up and GRAB her ear. (She was a pony so that neck was nice and short to reach easily.) She caught on pretty quick that her ear would get grabbed if she went for the bit. That took care of that!!
But normally, I would probably treat this the same way that I would treat a horse that dives for some grass when you are out trail riding: One hard smack on their upper neck with the end of the reins and a loud "NO!". That gets your point across.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.