Tell him/her no in a loud voice and clap your hands. That usually works for me
She's in the cross ties already and th reason I'm asking is because she wears out her right shoe!
Put them in the cross ties and let them stand while you go about your business. You have to ignore them, if you give them attention even if it's negative they still win. If they dont eventually stop have a spray bottle or little squirter gun that can shoot a stream like 10 feet and just squirt when they paw. If you are not walking up to them they are not getting the attention they want and will eventually stop because they do not like water in the face and not knowing where it's coming from. Do not reward them or take them off the cross ties until they have earned it a lot of people do not teach their horses patience and it leads to bigger more dangerous problems. Good luck!
You just have to be very firm about it so they know it's wrong, you can clap at them, give a little yell, give a firm but light tug on their halter, gently poke their shoulder.
One of the things that I do is clap my hand really sharply so it makes a loud sound. This is only when he is bored. If I am giving treats I give him a sharp "hey!" and then wait for him to stop to give the treat. Another option is a mechanism that goes around their ankle that stops pawing, I believe you can find it on horse.com! Good luck!