Hi.

I'm sorry if this has been asked before, I looked and didn't see.

Can the response of siege of winterfell (2 power for winning a millitary challenge) be cancelled by to be a Kraken ?

Thanks

No. The FAQ specifically forbids Agenda effects from being canceled.

What about Mance though ? he can blank them ?

The text on Mance specifically says he can cancel a North agenda. Ser Mandon Moore can come out of shadows during the challenge phase - no other shadow card can do this.

Mance actually isn't canceling anything. He's simply blanking the Agenda. Canceling a card effect and blanking a card that produces card effects are two completely different things.