We use a tool called a BALL JOINT SEPERATOR to get cranks off, does a quick easy job - every time. Just don't put them back on or you'll have to do it again! :shock:
If you don't want to keep any of the bits....angle grinder through the BB axle :twisted: I've done this on a couple of quality ebay purchases that 'included' a BB and crankset :roll: it's the fastest solution.
If its the driveside crank that wont come, take off the chainrings and see whether you can get a wrench on the BB cup. Unscrew the BB cup from the frame with the crank attached.