It's interesting to see how knowhow disappears when certain jobs drop out of the mainstream. Until the early seventies, cottered cranks were the norm, and shops stocked the 3 or 4 standard size replacement cotters. Removal was easy, you backed off the nut to the top of the thread, dollied the crank, and gave the pin one well directed blow. 95% of the time out it popped. Then you removed the nut and pushed it free. Most times the thread was OK, and if the cotter was in good shape you could reuse it. Otherwise you replaced both cotters, to ensure that the taper angle was the same, and the cranks sat 180° apart.

For those few stubborn ones, tapping the arm on the spindle as cny-bikeman suggested helped, but could be damaging to the BB bearings. Otherwise, cut off the screw, braced up the arm more rigidly to protect the BB, and got out the 4# mallet, and punch. No mechanic of that era could keep a job, if cotters got the best of him.

When training new mechanics, I found the biggest problem is that they were afraid to deliver a hard enough blow, especially if dealing with a mushroomed pin where a punch was needed. It seems people are afraid to apply full power with their other hand in line with the hammer. I solved that by having them hold the punch with Vise-grips, and freed of worrying about their other hand, most would nail it with their next shot.