Something prohibitory is meant to stop you doing a thing — a rule, a notice, a tax, or a court order whose job is “no.” A prohibitory sign tells you not to enter; a prohibitory law forbids an act; a prohibitory tariff can be set so high that almost nobody imports the goods. It is formal, often legal or official. Though actually, people mix it with prohibitive. Prohibitive often means “so costly you cannot do it.” Prohibitory is about the intention to block, not about the price by itself — even if a huge tax is the tool that does the blocking.