Thanks BurtW. However it is more than just the subsidy. The same concept applies to fees as well.
The protocol simply requires the value of the coinbase (reward to miner) to be LESS THAN the amount of fees + block subsidy. So imagine in the future when block subsidy is zero and fees and a block has x BTC in fees. A miner solves the block and has the coinbase set to zero. 12 BTC of existing coins will be permanently destroyed. Users will pay 12 BTC in fees and the miner won't collect them. This can happen at any time.
Lets look at a recent block.
https://blockchain.info/block-index/445116/000000000000000564bb70e7d7b56c9e0e399aea3c82ec84154f84aee94cc7cdThe current block subsidy = 25 BTC
The value of tx fees in block = 0.01678413 BTC
The amount paid to miner (see first tx) = 25.01678413 BTC.
So this is obviously a valid block.
If the coinbase was 25.0167841
4 BTC or higher it would be an invalid block.
However if the coinbase was 25.0167841
2 BTC or less it would still be a valid block and one or more satoshis would be permanently "deleted".
There are at least six blocks where the coinbase payout is less than block subsidy + fees, so some coins have already been destroyed in this manner although I assume this was due to a miner error rather than any malicious intent.
I am not particularly worried about it but the answer to the question "are bitcoins indestructible?" the answer is a definitive no.