You can remove the block reward as a soft fork, the consensus rule for Bitcoin and its derivatives is a maximum. Even if it wasn't, you can soft fork in a rule that the block reward should be spent to somewhere invalid, or paid to a particular person.
That doesn't work, unless 100% of miners accept your rule change forever (aka impossible). It would only take a single miner to keep mining using the old rules and this miner would reap the block reward / transaction fees. An old Bitcoin client would follow the chain created by this miner, wouldn't it?
