mouse

Is it that Bibi the cat has too many mouse-toys, and does not know the difference,
or is it the killer-instinct....??????????

yesterday Bibi came home with a real mouse... dead..but still warm.
pffft I had no mouse on his menu for that day, so I took it away.
This morning however..more worse, I heard a screaming sound...which I never had heard before.. looked..and there she was.. running towards me.
Bibi with a young-small-screaming-squirrel between his teeth.

Immediately I grabbed her in the neck, by which she was the one, who screamed and yelled
and so doing, openend her mouth wide open, so the squirrel, still alive, was able to run away.
I saw the squirell quickly hide himself under some ivy and saw that it could still run..
pffft.... I had no squirrel on his menu either... !

What can I do to kill the killer-instinct?