If you look around the surround you should see four holes, one in each corner these holes are the access to the locking system which holds the radio in place,if you cannot see the holes in question they could be obscured by a plastic covering which goes around the perimeter of the radio face which should be easily removed with a small screwdriver.
Once the holes are accesible you should get a basic metal coat hanger and cut it into four pieces say six inches long,place one in each hole and slide the radio out.
If at this point there is still no sign of it moving try pushing outwards/inwards on the metal coat hanger pieces (two at a time on each side)This should do the trick...also just to remember if you decide to put the casette player back in at a later time i.e if you are selling the car you may need the radio code as most radios these days lock themselves once removed from the vehicles electrical system as a security measure against theft.The code should be with the original manuals that came with the car,if not contact the main dealer that the car was originally sold by when new.