A good crank extractor SHOULD get them off. Whenever you put new cranks on you should always whack some copperslip on the taper and the inside of the crank, so as 1) it doesn't creak and 2) it's easy to get them off, which seems to be your problem now. If you really can't get it off, get down your local bike shop and get them to have a go at it. Hope that helps. Rich