to stop someone from talking or making suggestions, often in an unpleasant way

I tried to suggest ways in which the plans could be improved, but he slapped me down. 我试图提出可以改进那些计划的方法，但被他一口否决了。

to criticize someone or something severely, usually in public

This claim was slapped down by the chairman. 这一说法遭到了主席的公开批评。