I had the same problems with two of my boots. At one of my boots I put a piece of rubber, neonprene at the spot, the material they use for wetsuits. Worked fine with me. The other boot I didn't fix a thing, I only used better insoles so my feet couldn't move, my heels were fixed, and cranked the boots real tight. Hope you will solve the problems, otherwise your snowboarddays really sucks with this pain.
Greets, Hans.