So...let me get this straight. Your problem is your party thief might be trying to skim gold off the top in an evil campaign and you want advice about to stop the crime before it happens based on OOC knowledge alone?

...Yeah, there's something wrong here and I don't think it's your thieving problems. This isn't like a case of loot being divied up and then the guy stealing stuff that is rightly yours in your sleep, this is about the thief taking some of the gold out of some treasure he found before turning it over to the party to divide up into equal portions when it's still free game. If anything I'd count you lucky that he's not just taking it all and saying he didn't find anything.

More importantly, what's the harm in it? I've played a klepto-thief just once, and whenever I found a particularly large horde or something before the party did I'd squirrel away a couple gemstones or couple hundred gold pieces without telling anyone. I never spent them, I loaned them out to my party members whenever they were short on cash to seem like a nicer guy to them and picking up an extremely low-rate interest that compensated me for my own personal losses with legitimate cash. It also formed a bond with my party, even if it was just one of owing debts to someone which can be highly important in evil groups to keep it together since the lawful evil or neutral evil party members will at least think about the debt at times and the fact they have someone so free with loaning their own cash before turning on them.

In other words, I'm not agreeing with the other posters or your own decision that this is a problem because you don't even know if the thief will be successful or what he plans on doing with the money.