She couldn't wipe the smile off her face. [=she couldn't stop smiling]她忍不住要笑。

He thinks he's won, but if I get my hands on him, I'll wipe that smile off his face!他觉着自己获胜了，但如果我来对付他的话，他就不会这么得意了！

Wipe that smile off your face! This isn't a joke!别笑了！这不是笑话！