They should irradiate the coins in a nuclear reactor so that no one can ever safely handle them.
You're assuming that would have an effect. I tend to doubt it would. If the Coins are immune to change in the sense of melting or hammering or whatever, there's no reason to assume they can undergo nuclear transmutations either.
Now, you might just be on to a tactic, though. You probably can't make the Coins radioactive as such, but you could possibly put radioactive material
on them. Say, dip one in glue, then sprinkle it would radioactive dust. But over time that would fade away (as a rule, the more powerful the radiation, the shorter the half-life) and it could be removed by removing the sticky coating.
Or you could coat the Coin in poison, or corrosive, or something, but it could still be washed off.
Of course, making the Coins lethal on contact temporarily removes the risk of a new Denarian, but it has its own nasty moral problems, and the Adversary has plenty of time, at least on a mortal scale. The Fallen could just wait until the radiation fades away, or somebody washes the poison off, or whatever.
A more practical idea, it seems to me, and more moral, is Harry's approach. Take the Coins, encase them in blocks of high-density concrete, that sort of thing. Make them physically harder to touch at all.
But nothing will work for very long, by the very nature of the case. The Coins, to borrow a line from Tolkien, 'have a way of being found'.
