Be very sure that you want to do this. If you delete their accounts, posts will remain but will be unassociated with any accounts. If a user decides to return and creates a new account, it'll be very difficult to re-associate their old posts with the new account.
Personally I don't delete accounts, they are either left as they are or I remove/change details in them. The only accounts I do delete are those with 0 posts and content.