M.B.
I don't have any experience with this as my boys are under 2. But my nephew was sucking his thumb like crazy. They tried so many things. You know what worked?? They told him that if he sucked his thumb too long it would fall off. Then when he was sleeping she dipped his thumb in water with lots of green food coloring in it so that it dyed his thumb green. When he woke up he showed it to his mom, she said that it was getting ready to fall off because he was sucking it so much. He never sucked his thumb again - well, he would then he would catch himself and quickly remove it.
IDK if I would do that, but that is what she did and it worked for them!