I really wouldn't worry about deleting the accounts, you're talking [usually] at less than a kilobyte of data per member. (average in my dev environment is 435 bytes and those are accounts that haven't had things like signatures etc deleted). If you trust the moderators, then setting the action to delete the posted content is a good approach, that would have a greater impact on data usage than the account.
As you've noted, keeping the accounts there prevent the email address from being used again.