Why do you void arbitrage winnings? You don't allow arbitrage on your site? Do you mean he bet on double side to decrease the risk? IMO it is acceptable.
Hi jujugoboom, I think you confuse arbitrage betting with "matched betting". An arbitrator bets on double "sites" and he "doesn't decrease risk", he takes none. No matter the match result, he will always have a small gain. Different sportsbooks have different policies about arbitrage, we are against it.
How can you know that it was arbitrage betting?
Do you have the data of other betting sites in order to collerate both?
That's impossible to know for the gambling site...
Detecting arbitrage betting is rather easy for an operator as there are pretty clear and simple indicators. You could actually google about it and its detection.