if the developers can't detect exploits like this then they need to find them manually. i would suggest the following. find the cheapest spammers in game, the ones selling for around 1 dollar per million, those are the ones obviously exploiting a bug. then the developers should buy credits from them and find out which bug they use to generate those credits. then fix the bug and ban them after you learned what the exploit/dupe is. but the key is to use them to find the exploits and fix the exploits.
they just need to focus on the really cheap spammers, those are the exploiters, find their methods and fix them and then ban them.
this is costing them a ton of cartel coin sales and will lead to a lot more inflation if not addressed and fixed.