No Smoking
If smoke is prohibited you probably shouldn't smoke. Aka no smoking, okay?
きんえん
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
ここが禁煙スペースだってこと、忘れてるわけないじゃない。
How could I forget that this is a no smoking area?
毎週月曜日は禁煙の日って決めているんです。
I've decided to make every Monday a no smoking day.
母親に禁煙するように言ったんですが、逆に母親から、禁煙したらおそらく気が狂って、父親と離婚することになると言われました。
I told my mom to quit smoking and she told me that if she did, she would probably go crazy and my parents would get divorced.