If the player is any good, you simply cannot punish him, even if you do he'll just retreat instantly under tower and heal most of the damage. After level 6 you're screwed up, if he gets a lead (which he will) god forbid you have less than half your HP (why is an execute doing so much goddamn damage anyways, and it's an AoE).
I can't think of another champion that's able to get pentas so consistently. He's the second most picked champion in ARAM (the first one is Ez, which may be a pain in the ass, but you can actually punish him) so you'll be seeing him a lot.
Just bring bans back, nerf Pyke, or make two separate modes: normal and draft ARAM. It wouldn't take much effort.