Just use some muscle. It's been a while but I believe that's a rather large nut keeping that on there. Wow. I really don't remember if that puppy is even reverse thread.
That right there could be why it won't budge.
In any event,there's a nut holding it down.
Flip your bike upside down so it sits on your handlebars and seat.
Loosen your rear wheel nuts still on your bike but leave your coaster lever attached to your frame. That will give you leverage. Then with channel locks or even with a pipe wrench if you can get it in there,try loosening it.
Then when the nut's off the arm just falls out.
But it could be reverse thread.
What's the verdict on this?