THIS is how it was done when cottered cranks ruled.

You'll hammer the crank out in a single blow, but the key is you have to support the arm on a rigid equivalent of an anvil. If you have a heavy vise on a solid bench that'll work, but my preferred method is to stand a 1" water pipe on a concrete floor or directly over a brace on a wood floor, or someplace equally solid.

Stand the pipe up, and have a friend hold the bike up supporting the crank on the pipe with the head of the pin inside the pipe. Take the heaviest hammer you own, line up your shot (take a practice tap if necessary) then deliver the best shot you have in an attempt to drive the pin down the pipe, through the floor and on to China.

Keep in mind that multiple blows, or hammering on an unsupported crank will only mushroom the screw end. But a single blow on a supported crank has too much energy for the pin to absorb by mushrooming, so out it goes. (though it won't get to China).

If you're not a master with a hammer, or if the screw is already bent or mushroomed, take a hacksaw and saw it off flush (use a washer so you don't mar the crank. Hold a punch in a pair of vise grips, and drive that. (the vise grips are because most people are chicken and don't hammer full power if they're holding the punch).

Remember, this is a one shot operation, so do whatever it takes to get your head around the concept and swing for the fence.