How? It would be extremely bad business to ban a big winner. Big winners are an outlier and most casinos know this. Big winners are the best thing to attract more customers.
And there's a pretty good chance the grand winner will continue to bet in order to multiply and earn even more - rising the chances of losing it all again so the casino is never really on a loss. Happens more often than not 
I guess every casino applies as many strategies to keep you addicted and to make you come back, online or not. If you owned a casino, you'd do the same as your only interest is making a profit out of it. Online casinos have their own tricks too. One thing that kept me addicted to PrimeDice years ago was their rainbot and the faucet. I spent too much of my time for a faucet of 200 satoshis (BTC was under $300 at the time) and yet at one point I did get convinced to deposit too.
Hence, you could write a "Online Casinos Decoded" thread too as there are a lot of tricks websites use to keep you in touch with them.
The "maze" trick is used in some shops too, if you've ever noticed. There are lots of shops out there created in such a way that you have to see everything in order to get to the exit.