Can be done - depending on the car, very easy to medium difficult.
Remove the center bolt and the clutch "lining" pops off the compressor
driveshaft nose.  Remove a circlip and the idler pulley is supposed to
slide right off with a little persuasion.  

 I just did one on a Ford Van where the compressor is under the
engine, and the job itself wasn't bad - but getting there, however...

 Because of space concerns getting in there with a standard three-jaw
puller, I had to warm up the welder and custom-build myself a puller
tool to get the idler pulley off the front of the compressor.  Once I
had leverage, it went fast.

    --<< Bruce >>--