I had this problem in Archeage too. The best way I had of overcoming it was putting the rules clearly on the walls of the casino and using a public dice roll for the result. Business was slow at first, and met with a tonne of skepticism, but after awhile my reputation grew and people focused more on the gambling and less on the scamming.
I think the machines may be hard to prove transparent without showing the code (and even then I could use a different code in game). But if someone realises you're scamming the show is over. You got a nice sum but it will be harder to get more. If you have a good reputation and provide a fair gambling arena, people will come. Of course gambling is profitable for the house so I don't see any reason to scam anyone for a short term buck.