Surcouf, I have had success by using a long chisel or saw to cut the clamp on the hose. Once cut try twisting the hose to see if it will loosen, if not remove the screws from the exhaust flange and apply more twist and some spray lube into the gap.
With some flanges the base is large enough to open the hole in the transom a bit and pull the whole thing out. A small hole around the flange can also be used to chisel the clamp or clamps. Just be sure to reseal the transom.
There may be another flange available if the one on yours won't cover the larger opening.
Dave Neptune