The chances are that it is glued together or possibly welded at various points around the perimeter. Make sure you've removed all the screws (some could be hidden under labels). A credit card is also useful for prying into the gap between the halves and would do less damage to the plastic than a knife. If you can separate the halves even a bit, you might be able to see where the snap clips are and push in and upwards to release them. Another possibility is to heat the edges of the control with a hair drier. Most glues soften with heat, so it may help to free the halves. Other than that your only option is to cut the control using a junior hacksaw or maybe a toothed cutting disk and Dremel rotary tool.