Only the player has the authority to define their code - - the agreement they made with their god. I don't see anywhere in the rules that I, as DM, am the final arbiter of such, even in the sidebar where it recommends a discussion. I agree, you should always discuss things with your players so that you're on the same page as to understanding.
But, no, if you're going to make paladins adhere to an stringent external code of which you, as DM, are the only arbiter, then you owe more to the player that agrees to this than to any other player at the table. You've constrained their ability to play more than others, and that should only be done with commiserate compensation. So, what is it you do to make paladins better than other classes that don't have you as DM telling them how to roleplay properly?