This will only work if your characters are not yet equipped with armour. This is very simple, all you need to do is buy just one armour for the character you want this to work for and make sure he has no other armour equipped. Once you have bought the armour of your choice save your game, then equip the armour and then load your game. You will notice that your characters defence stays the same as if he has still got the armour equipped but he has not, then simply repeat the process and there you have it, infinite defence increase.