I wanted take mine off and had used polystyrene cement (so a melted bond), so freezing was not an option. I just used a very sharp (new blade) exacto knife. Very slowly I worked around the edges and cut it off but slowly pushing the blade between the sponson edge and the tank side.
Then I built it so I could re-attach the side sponsons with magnets. Used the spare small access panels (with a magnet on the other side) to cover the hole and did some sanding and clean-up. Then with putty and a magnet I made it so the side sponson could be added if I wanted.
Here is the tank while the putty I filled into the side sponsons dried (that is how one magnet is held in place), with cellophane so it would not stick. On the other side of the small access hatch is the other magnet.
With Lascannon and Heavy Bolter Sponsons:
With Heavy Bolter and no side Sponsons:
Hope that helps,