Shall I tell John about it?的否定回答的答案是,No,you needn't.和No,you don't have to.可我认为应该是：No,you can't.

Shall I tell John about it?的否定回答的答案是,No,you needn't.和No,you don't have to.可我认为应该是：No,you can't.
Shall I tell John about it?的否定回答的答案是,No,you needn't.和No,you don't have to.
可我认为应该是：No,you can't.

Shall I tell John about it?的否定回答的答案是,No,you needn't.和No,you don't have to.可我认为应该是：No,you can't.
No,you needn't.和No,you don't have to.语气较为婉转,而No,you can't.则显得比较生硬,欠礼貌.

You cant的确不适合。

如果你想警告他就说，no, you can't,通常后面会加上原因：我哥哥会杀了你的....

you shall not or you'd better not

书面做题时一般都没有No, you can't这个回答。提建议是要用委婉语气，这个非正式语体上可以