Can a bitcoin be destroyed?
Sorry Alex, but you're wrong on the question being asked.  It's trivial to put bitcoins "beyond use" so that they can never be spent again.  All you would have to do is send the bitcoins to a made up address, which no one would have a key to.  This has already been done, since coins have been sent to addresses which are almost certainly unowned (like the lowest possible hash that is a valid address).

So specifically in your situation, the BTC2 people could use a public method of creating an address that no one owns the private keys to, then design their system to credit people who send BTC1 bitcoins to that address.  One reason someone might want to do this despite the open-source nature of Bitcoin would be to create a new, incompatible protocol that grabs some of the user base of BTC1 without forcing anyone to leave, allowing the two to coexist and compete quite fairly.