Here's a rough draft of yet another argument I'm toying with that's in the same vein as several others I've discussed here recently:
Suppose for reductio that it's metaphysically possible that a necessary being exists, and that this being is the god of classical Anselmian theism. Let's follow Plantinga's claim here that such a being has the property of maximal greatness, where: (i) a being's maximal greatness entails maximal excellence in every possible world, (ii) maximal excellence includes the classical attributes of omniscience, omnipotence, and moral perfection, and (iii) omnipotence includes the capacity to create or sustain concrete objects distinct from itself without a material cause. Therefore, if it's metaphysically possible that a maximally great being exists, then such a being exists in all metaphysically possible worlds. Therefore, a maximally great being exists in all metaphysically possible worlds. By the above conception of maximal excellence, for any world W that contains a universe of concrete objects distinct from God, if God exists in W, then God originates or sustains the universe in W without a material cause. But the origination or sustenance of any such universe without a material cause is metaphysically impossible. Now a universe of concrete objects exists at the actual world. Therefore, the god of classical Anselmian theism did not originate or sustain the universe that exists at the actual world. Therefore, the god of classical Anselmian theism doesn't exist at the actual world. But this contradicts the above line that he exists in all possible worlds. Therefore, the existence of the god of classical Anselmian theism is metaphysically impossible.
The upshot is that Plantinga appears to have been wrong: the crucial premise of the modal ontological argument -- viz., that a maximally great being (as Plantinga understands that notion) is metaphysically possible -- is contrary to reason. For we've just seen that the notion of a being that is the creator of all other concrete objects distinct from himself is on a par with the concept of a being that is the creator of round squares, as both entail a metaphysical impossibility.