Can a bitcoin be destroyed? Its the hypothetical future and BTC2 is being designed. Everyone loves BTC2 and no-one wants the old BTC1 any more.

People who have BTC1 would be annoyed that their assets are suddently worth a lot less because there's no demand for their BTC1 any more, so the designers of BTC2 want to include a simple way to convert their BTC1 into BTC2.

For this to work, the BTC1 world would need a way for coins to be put beyond use within the BTC1 world, in a way that everyone can be reasonably sure that no-one can use the evaporated BTC1 assets any more.

Does such a mechanism exist? If not, could/should one be added?
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.