I see this asked over and over, and I'm tired of typing it, over and over, so I made a pictorial.
it's so easy, a child could do it. (I should know, I rebuilt my first coaster brake hub when i was ten
step 1) set the wheel in your lap, coasterbrake arm up, and set the arm in the 4 O-clock position.
step 2) put your wrench on the nut in the 7 O-clock position.
step 3) hold the wrench in your left hand, and using a hammer (or a pipe or a bigger wrench as a lever,) whack (or lever) the brake arm towards the wrench (clockwise,) while moving the wrench counter-clockwise.
it should loosen right up.