if the compressor starts to turn, then locks, then spins again, like I think you mean, then the thing is shot...it should turn smoothly all the way around(but with some resistance)...if it stops, you have a compressor that is coming apart...
long story short, get a new compressor, and change the filter drier while the system is apart...you'll thank yourself later.
also, with the engine off(or on, for that matter) and the AC turned off, the plate SHOULD slip...the opposite of what your first post says...the clutch should have an air gap when turned off of something like .025" or something, and then when you hit the AC button, the coil energizes and pulls the clutch inward, locking the pulley to the compressor shaft.
