I had this problem a few months ago with one of my bikes. If you want to keep the cranks then go with what the other posters said. If you're replacing the cranks and don't care what happens to them then I suggest cutting them off or pulling them off. If it's the non drive side you can get a crank puller from an auto store which has either two or three arms which fit around the crank with a large screw on bolt down the middle. Turn the bolt and the arms will lift and eventually pull the crank off (kind of like how a corkscrew works; sort of). If it's the drive side you will have to hacksaw your way through the crank arm. Aim to get the blade down one of the four sides of the BB spindle (not straight down the middle). Saw down beside the spindle. I managed to hacksaw about an inch down beside the spindle before getting a chsiel and rubber mallet. A couple of whacks and the crank split in half. Pulled it away and installed the new cranks. A little frustrating but it can be done. Good luck.