Would it be possible to destroy gold?
Other than a nuclear reactor, the only other chance is to dump it into a volcano. Having a much higher density than magma, it will just fall through until it hits earth's mantle. Then it's really gone. OK, dissolved and diluted in the Atlantic is also quite safe.

P.S. Warning: If the bullion gets stuck in the volcano, which later explodes Krakatau-style, your gold brick might land in someone's front lawn. Making a hole five meters deep, but it may be salvaged easily.