How can coins be destroyed for good? how can you ensure these coins can never be used again?
You can "burn" coins (not exactly the same as destroying, but probably what you are looking for) by sending them to an address that is unspendable.
https://en.bitcoin.it/wiki/Proof_of_burnYes, i think that is what we need, but how to create an address that is unspendable. This is the main issue. We need something that does not need trust that in turn leads to doubt and rumour. We were looking for a way to destroy them beyond all doubt. I guess this is not actually possible at the stage we're at now. We will have to rethink this.