Why is では used here? What does it do to the meaning of this sentence?


I came across a sentence that has puzzled me.

> 今度ばかりはいたずら **では** 済まされません｡

Why is **では** used here?




～ **で** 済む is a set phrase that means "to get away with ～".

> これで済んだと思うなよ。  
>  You won't get away with this.
>
> ここから落ちたら怪我では済まない。  
>  If you fall from here, you can't just get hurt and survive.

いたずらで済ませる means "to get away calling it a prank", "to forget thinking of it as
a prank", etc. は is used because it's a negative sentence (see: [Why is the
topic marker often used in negative statements (ではない,
～とは思わない)?](https://japanese.stackexchange.com/q/1077/5010)).

