What are substances that cannot be decomposed to any simpler substances?

If you are not concerned about atoms, electrons, neutrons, protons and other sub atomic particles, but confine to chemical substances occurring in nature or man made, the substance that cannot be broken into simpler substances is an "element" (usually enumerated in a periodic table).
A chemical element cannot be decomposed into a simpler substance.
Elements. They are defined as substances in their simplest form and cannot be broken down or decomposed into any simpler substances. They include all elements in the periodic table of elements.