Nah, popps really does have some good points quite often, but he lets the spirit move him a little too much in some of his replies.
I sorta am between the original poster & popps on this one.
Here's how one can do this to minimize the effect of the flood of extra vet rewards, for a permanent NPC...
a. Have the NPC check the the number of vet rewards already claimed by the account. That will be the number of potential exchanges one can make.
b. Exchanges can only be within the same category. For example, one can only get back a monster statue, for trading in a monster statue, a house add-on for a house add-on, etc.
c. Have soulstones to where only empty stones can be traded in (that will keep most of the existing orphaned stones from being traded in), and if you pick a soulstone as a trade-in reward, you have to undergo a 3 or 6 month cooldown, before you can trade for another soulstone.