At the moment there is a limit of 50 MH/s per worker. If someone has doubts about this - please feel free to test it. One can rent a rig (for example from betarigs) that has hashrate above 50 MH/s and try to mine with it. You will see that this worker will give you around 50 MH/s.
Miners that have total hashrate above 50 MH/s are using multiple workers and that is why you see some miners with 300 MH/s and above. We will let the pools run as is for now. We are considering implementing limits per IP. We might implement per IP limits in the next 24 hours.
This won't work. People won't have the same IP when they rent rigs obviously. What you should have done is not let anyone go over 50mh/s with all workers combined and only make one pool, so people can't mine on two different at the same time with rented rigs.
Edit: or have two pools and not let anyone sign up from the same IP.
Do you really believe that using multiple IPs is a big hurdle? I don't think that it is. Statistics page will look nicer as there would be no big miners but apart from that I don't think that it will make a huge difference.