狗改不了吃屎

old habits are hard to change (idiom)

It is hard to change one's essential nature (idiom). You can't change who you are.; Can the leopard change his spots? (Jeremiah 13:23)

rivers and mountains are easy to change, man's character much harder

it is easier to change mountains and rivers than to alter one's character (idiom); you can't change who you are

lit. no ivory comes from the mouth of a dog (idiom); fig. one does not expect fine words from a scoundrel

lit. rotten wood cannot be carved (idiom); fig. you can't make a silk purse out of a sow's ear