As of 2014, miners that successfully mine a block are entitled to 25 bitcoins.
Can miners refuse to receive those 25 bitcoins, thereby reducing the total number of bitcoins that will ever be mined?
For the block to be valid there must be a coinbase transaction but it is only checked that it spends at much the reward and the fees generated in the transactions included in the block. So it is possible to generate a valid block with a coinbase transaction spending less than the available coin and thus destroying coins forever.
But you can always send the reward to a random address in the hope that no one will ever be able to find it's private key and that it does not belongs to anyone.