Typically this is caused by the cassette digging into the splines on a Shimano freehub (Campy has deeper splines that are much less likely to be gouged by the cassette). To get the cassette off you need to turn the problematic sprockets in the direction that the freehub rotates freely (CCW with the cassette facing you; opposite the direction torque is typically applied when pedaling). To do this, you can hold the larger sprockets (the largest three are usually riveted together somehow) to keep the freehub from turning and turn the medium/smaller ones CCW. If that doesn't work by hand, you can use two chain whips: one to hold the larger sprocket cluster and the other in the opposite direction to free the smaller sprockets from the freehub one by one.
On Oct 2, 2012, at 4:16 PM, Riley wrote: