WordNet sense 1 (severely simple):

severe, stark, stern

WordNet sense 2 (not elaborate or elaborated; simple):

plain

WordNet sense 3 (characterized by strictness, severity, or restraint):

strict, nonindulgent

WordNet sense 4 (practicing great self-denial):

ascetic, spartan, ascetical

WordNet sense 5 (sparing in consumption of especially food and drink):

abstemious