Horse still pulling reins at standstill...what to do?

i have a 4 year old arab/qh. she's very well behaved and does anything i ask her to, she is highstrung and spooky no matte rhow much i work with her. it took a few weeks to teach her to stand still, but now all i have to do is say woah, and she stops on the dot, but she yanks the reins. i ride her in a d-ring snaffle and she's had no problems with it, no teeth problems, nothing. when she does pull, ill smack her on the shoulder and she'll stop for the time being, but then she'll try again. ive been trying to break this habit since july and ive been very consistant with it, but she still does it. i used to ride an 18 year old full arab who did the same thing...is it an arabian thing? it needs to stop if im going to show her in the spring.