Something PROHIBITIVE (excessively costly or restrictive) prevents a PURCHASE from happening. Something imperative (absolutely necessary/commanding) compels one to comply. Both pairs = [adjective describing a force] : [action it drives or prevents]. Note: the pattern here is [adjective] causes/prevents [action]. 'Preventive:heal' works partially but 'preventive' is more general. 'Imperative:comply' most closely mirrors the cause-effect relationship.