WordNet sense 1 (firm in purpose or belief; characterized by firmness and determination):

resolute

WordNet sense 2 (unsparing and uncompromising in discipline or judgment):

severe

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

strict, nonindulgent

WordNet sense 4 (practicing great self-denial):

ascetic, austere, ascetical

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

abstemious