What is a substance that cannot be broken down further into simpler substances?

The conventional answer is "an element".
One contributor proposed "atom"; another noted that "an atom can be broken down to its parts which include electrons, protons and neutrons. neutrinos are now the smallest substance [sic]"

However, it is doubtful that protons, electrons, neutrinos, neutrons and the other atomic particles should be considered "substances", because they can not exist in pure volumes of visible size.

Another less confident earlier contributor wrote correctly "Elements are the answer you're looking for. While they can be broken down into the atoms they're comprised of, elements are the most basic building blocks of all substances. [punctuation slightly improved]"