I have told to the support I use a VPN but he don't care.
I think that was the reason why have detected you using multiple account. Most likely they have checked your IP address, and usually it would happened if you had used VPN. Perhaps some other users used same VPN or IP address that you had used. Is it against their terms & conditions? I am not sure about it since I had not read. I don't think they will scam this kind of small amount since they have solve more issue with more big amount. Try to discuss their ANN thread as coinlocket$ already mentioned, hope your problem will solve.
Since OP admitted that he is using a VPN,Sportsbet has a good and valid reason to ban you, they have campaign I have seen their signature campaign several times and they are one of the top betting sites and I doubt if they are on a scamming business for just a small amount you should lock this thread for now and go to their official thread to discuss this issue do not be to hurry to accuse.