If the user of a tinker tool has fiery enchant on it they will be able to mine iron and gold ore receiving the ingot (gold/iron ingot) and the ore (gold / iron ore (block)) meaning that they can replace then mine the ore and continue this process. This is used by some people to make slow money although most people simply mine as it is faster and more profitable. However when visiting a plot from Ajar's town i noticed that the iron ore in towny when tried to be broken would drop the iron ingot and obviously with world guard and towny not break meaning someone could keep mining and make thousands of ingots. At the time before that I was mining but I fortunately did not sell any iron or gold which are slightly less valuable and are the only ones where this would work on, it can work also on things like diorite and andesite and many other blocks but those have basically no value.
I would suggest disabling the fiery tinker tool enchant for now or if thats not possible temporarily remove the sale of gold and iron ingots at the admin shop.