I don't know how useful my input will be since normally my gelding ties like a gentleman, plus I am in no way an experienced horse trainer, however...
I think it's important to assess the horse's reason for pulling back. Is he pulling back out of evasiveness, or fear? It's unacceptable either way, but the situations require different approaches. You cannot stop an animal from being afraid by punishing fear.
For example, if my gelding starts to pull while evading fly spray, having his hooves messed with, or something else that he knows better then to try to avoid I immediately correct him, make him stand square and continue whatever it was that I was doing. I'm not very nice about it, either - this behavior is what I consider to be unacceptable and disrespectful. Once I'm done - on my terms, not his - and as long as he's behaved himself, he gets a pat and some verbal praise for being a good boy. I also try to make sure we end things on a good note so I CAN praise him - when training any animal it's important to set them up for success!
On the other hand, if something happens to spook him (this is fairly rare) and he starts backing up out of fear I IMMEDIATELY start trying to redirect his attention to me. I accomplish this by yelling something like "HEY BUCK TAKE IT EASY EASY BOY" or something like that - the words aren't what matters, the tone is. No fear, no anxiety, and no anger because that would escalate the situation and what I DON'T want is for him to get into the cycle of pulling back out of fear, hitting the end of the rope, getting more scared and continuing to pull and so on. Just loud, to get his attention. Once he stops focusing on being scared and starts focusing on me (usually takes a few seconds) I settle him down, stand him square again, and go back to work. I try to do so in a very no-nonsense manner so he sees that I am not frightened and there is nothing to be frightened about.
This is how I handle the issue of pulling back, and it's worked very well for me and my horse. I don't know how well it would work with another horse.