I've broke 2 from it now, and it is NOT easy, or fun, and you have to have really good timing. No, you cannot fix it when they are roaming free etc, but in hand, while I'm nearby and they're tied, in harness, it's potentially dangerous and needs to be stopped.
My first step is some serious patience pole time. They stand tied if I'm in the barn pretty much - even if that's hours. They don't go do fun things until they are just STANDING there - not playing with their mouth, pawing, etc. chronic rope chewers I put a grazing muzzle on until they 'get it.' My one guy pawed on the hitching rail because he was antcipating something (workout, trailer ride, or the much dreaded bath) so I tied him there, found a comfy lawn chair, and a good book for an hour or two and completely ignored him. Psyched him out the first few times. Now, he just STANDS because that's what we do at the hitching post no matter what - just stand there!
My filly was not so easy. Patience pole time did not work. Out of frustration I pegged her right in the knee mid paw with a wood block, and continued on like it never happened. She had no idea it was me and seriously thought god had smited her for her actions. Look on her face was oh so priceless. Has cut way down and respects a 'no' much better now.
For pawing in hand, the only thing I've found to work is to wear a big heavy boot, and don't kick them in the leg but pick your foot up right as they go to do it so they hit you with the middle of their cannon bone. You cannot do it out of anger or boot them (for obvious reasons) but it makes a real impression when done in a 'matter of fact' way. The key is to teach them there are unacceptable times to paw, not try to teach them not to do it.