One should not let bad behaviour go unchecked.

对错误的行为， 不能放任不管。

《新英汉大辞典》

You must not let the rumour go unchecked.

你们不能让这个谣言传播开来。

youdao

If we let these problems go unchecked we will risk losing the trust and support of the people.

如果我们不能解决存在的问题，任其发展下去，人民就不会信任和支持我们。

youdao

And we have learned that we cannot stand by and let such atrocities go on unchecked with impunity.

CNN: By Candy Crowley/