Sentence:  “It just shows you what women are like. A frightful sex, Bertie. There ought to be a law. I hope to live to see the day when women are no longer allowed.” “That would rather put a <WSD>stopper</WSD> on keeping the human race going, wouldn't it?” “Well, who wants to keep the human race going?” 
Meaning of the word marked with <WSD> tags:  Agent noun of stop, someone or something that stops something.