Uh, I think it’s pretty apparent that you have to cut off their arm with the mark to fight immortality. The contract was formed using their arms as payment which means when they turned demon the arm that grew back is demonic and probably has a mark which also allows you to travel through the Abyss.
Also would maybe add an extra layer to Shanks giving up his arm for Luffy since what if he had demon powers at the time due to being forced to and it was his way of cutting off all ties to his family because he finally realized that Luffy might be the savior and there won’t be any need for him to try sabotaging Imu‘s rule from within?
