Gragas vs Nasus top who wins?

You would think Gragas right when you can zone him from low minions with your Barrel Roll Q and when he goes to stack with his Siphion Strike Q you can knock him back and stun him with your Body Slam E denying him both farm and stacks right? I forgot months ago i stopped playing Gragas and Sejuani top for the same reason when agains a Nasus if presses Q and starts his Siphon Strike and and has already Clicked on a target if you knock im back and stun him no matter how far he is from the minion champion ward etc. The action, visual effects, sound effects, etc everything but the actual swing of the cane will go through negating the counter play of denying stacks and stopping his damage. As far as I know it also happens with Sejuani. No matter how you beat Nasus into submission early game he wouldnt have lost much strength because by trying to stop his stacking you actually garentee his stacking when there is a chance he could have missed on his way to get to the minion.