Why not just take the scorching coffee away instead of hitting them? There are alternatives, and without using violence a child will still learn not to grab hot liquid.
She had a firm grip on the cup, im surprised she didn't burn her fingers! I tried pulling her hands off but she kept grabbing so a small tap on her hand was all that worked. If i had of stood up that would have given her a chance to pull it on herself. I do however now drink my hot drinks at the kitchen bench. Of course i should have done that to begin with but hey can't change the past and i also learnt from it.