Check the Super Searchalator for "Terrifying Turban of Death" and you'll find a post that describes in detail how to remove it.

Basically, you're going to need patience, a ton of boiling water and several old towels to wrap around that agitator spline on top. Liquid Wrench beforehand is a very good idea. You'll also need someone with a good back and good upper body strength. This isn't easy and what happens usually, is that the activator will slip off of the "spline" which will remain on the inner post.

I forgot my own post! It's the "Boiling Turban of Terror". Go to post #26896