I suppose you'd need to cut off the upper part of the arm. But that might still leave the lower arm too long, can't say about that. Perhaps a better alternative would be to swap the entire arm for something else, maybe a Wargames Factory arm. The latter are decent and often available for cheap from bitz shops.
It's obvious from the pic that even a good paintjob can't hide their deformities, unfortunately. They do seem to have improved with the Enforcers.