The easiest way to destroy the value of numismatic coin would probably be to clean it. A coin at the highest level of grading would be "destroyed" (lowered to a less valuable rating) by simply picking it up outside of its sealed, graded holder.

There are plenty of ways to destroy it physically (throw it in a forge, bend it with pliers, etc).

I'm a bit more concerned that a scammer wouldn't destroy the coin per se, but make a claim on authenticity, causing me to lose my money and also the coin.