What paper(s) or theory(s) describe or prove that the elementary particles that we have determined today cannot be made up of smaller more fundamental particles?
One of the questions under investigation in the data being gathered at LHC is the search for compositeness of quarks and leptons. They gave limits for quark compositeness from the data of 2010.
So the answer is, it is an open question under investigation, though not popular with the theorists.
There is no such theory. We treat the elementary particles as elementary simply because we have never seen any evidence of them having a substructure.
I suppose someone might have published a paper claiming that they must be fundamental (after all, there are a lot of papers out there), but the vast majority of physicists do not take such claims seriously.
This answer, though somewhat dated, comes with the best credentials: "The question is still open experimentally, but theory and experiment are pointing more than ever before to the possibility that we have discovered the 'ultimate constituents'." — National Research Council (U.S.), Elementary-Particle Physics Panel (1998), Elementary-Particle Physics, National Academy Press, Washington, D.C., p. 23.
But are the "ultimate constituents" pointlike entities, or are they formless?