coaster brake issues

..are you sure your torquing the nut anti-clockwise? If you can, slip a cheater bar over the brake arm and a wrench on the nut. Double check that your not actually tightening. It's a right hand standard thread. A little wd-40 or other penetrating oil might not hurt either if this is an old unit..

take the wheel off and set in in your lap with the coaster brake arm at 4 o-clock.

put a wrench on the nut at 8 o-clock

hold the wrench in your left hand, and beat on the coaster brake arm with a hammer, toward the wrench. one or two good smacks oughta do it.

or put a piece of pipe over the arm, grab it with your right hand, the wrench in your left, and push your arms together.