/jp/ - Otaku Culture

I'm a little lost on to points here
>「そう」だという確信があったから
Can this be interpreted as "The resolution (for) that"
>盾の存在を記憶から消すより他なっかた
Is this "Didn't take other option than erasing the shield from her memories"?

Dunno if this is the right place. I don't get the part where it says "より他なかった". Up until that part she's talking about the moment where she saw the witch, she erased the existence of the shield from her memories. But what about the rest?