• 禁止きんし — prohibition
• 禁煙きんえん — no smoking
• 禁酒きんしゅ — prohibition of alcohol
• 禁断きんだん — forbidden, taboo
• 禁物きんもつ — taboo thing, something to avoid

The character 禁 combines two 林 (forests) over 示 (altar), indicating a sacred area where entry was prohibited. Over time, it came to mean “ban” or “prohibit” more broadly in religious and legal settings.

この場所ばしょでは 喫煙きつえんが 禁止きんしされています。
Smoking is prohibited in this area.

先生せんせいは 試験中しけんちゅうの 私語しごを 禁止きんしした。
The teacher banned talking during the test.

The on-yomi キン is used in all common words like 禁止 (きんし) and 禁煙 (きんえん). There is no commonly used kun-yomi.