And I would have you believe, my sons, that the same Justice which punishes sin may also most graciously forgive it, 

and that no ban is so heavy but that by prayer and repentance it may be removed.

麻烦老师讲解一下按照字面意思，把but=except的话，no ban is so heavy but that by prayer and repentance it may be removed.如何翻译呢？

例如：除了祈祷和悔改可以免除罪恶外，no ban is so heavy 这里应该如何理解和翻译呢？

类似：

There is no reason whatever to suspect foul play, or to imagine that death could be from any but natural causes

这句也是no...but...结构吗？麻烦老师也讲解一下，谢谢！

主要是no...but...结构，but=except, 那么如何理解翻译的问题，麻烦老师点拨一下，谢谢！