Of COURSE she needs that size of afghan! What could you be thinking?
About your problem: First, I would give her her own ball of yarn. You can offer this to her when she attacks yours.
My cat Miki was already 15 when I started knitting, so he just wanted to rake whatever I was working on out of my lap and dig in it.
So I knitted him the Royal Pet Afghan from Lionbrand out of 100% scratchy wool. This tends to distract him. Sort of.
The pattern is here (needs free registration): Free Knitting Pattern: Homespun® Royal Pet Afghan
I also made him Wendy's cabled catnip mouse. Miki cares nothing for the mouse, but your kitty sounds like the right age. Link to her pattern: Catnip Mouse Pattern
Another solution is just to go ahead and bind off now and give the afghan to your kitty.