If you pound the crank enough, you will damage or break the bearing cups. Can we assume that you did not use a support block under the crank cotter before you pounded on it?
You are experiencing the intense feeling of "COTTERS SUCK!".
Back in the days of cottered cranks, the pro wrenches had cotter presses to remove and replace cotters. In today's world, there are some tricks you can use to get it out, but in my humble opinion, the crank might be hosed.
First, if you can get one of the pedals off and remove the stuck pedal and axle, do that first rather than banging on the crank while it is on the bike.
If the stuck cotter is on the left crank side and you can't remove the crank/axle from the bike, you can try this; Take a 2 X 4 and cut it so it will fit under the crank arm by the bottom bracket. Cut a "V" notch into it so the cotter pin head rests in the recess of the 'V" notch.
Put some naval jelly or oxolic acid around the rusted cotter as much as possible and let it soak overnight. Rinse, then spray with Liquid Wrench over night.
Then, it's Hammer Time. Use a punch and a hammer to knock out the cotter. A broken pedal axle makes the perfect punch tool. Don't even think of using a typical nail counter-sink punch. That won't work.
When you it the punch/cotter you have to do it with a really good sharp whack - don't ***** foot. A heavy hammer is better than a light one.
If you are lucky, she will break free.
Last edited by mike; 09-27-08 at 08:20 AM.