WordNet sense 1 (break a piece from a whole):

break, snap off

WordNet sense 2 (break off (a piece from a whole)):

chip, come off, chip off

WordNet sense 3 (break a small piece off from):

knap, cut off

WordNet sense 4 (interrupt before its natural or planned end):

cut short, break short

WordNet sense 5 (prevent completion):

discontinue, stop