Two of our ponies have been really bad about getting their tongues over the bit when using a snaffle or similar. Last year I changed one of the ponies to a grazing bit . She learned very quickly she was not going to get her tongue over the bit. She started giving me her attention, instead of fighting the bit. I used it one her for 1 year. Then three weeks ago, I put a snaffle back in her mouth. Problem solved. No more issues. Now on to the next naughty pony 
Sometimes it is more about breaking the habit!
You can take a shoelace and tie the tongue down as well
