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 miner 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.
You say IP in your post
Big farms/ASICs?
Hashing power of each worker and IP will be limited to not more than 50 MH/s (see the poll) by limiting the number of shares that a worker can send to the pool per unit of time.