What's more, we know that hell will happen and will last forever and involve physical punishment, because God has said that it will. We don't want to allegorize those passages away to make ourselves feel better--hell will be physical suffering as well as spiritual suffering. God is capable of doing this, since he is "the Source of all that is wonderful in all natures whatsoever."
But if the fire of hell is material, how will evil spirits suffer in it? The answer is that 1) the fire is not just physical, it's also spiritual; 2) even now those in hell are suffering without having a body--the structure of the spiritual world is such that the physical world can affect it as well.