I can create a new user and as long as it isn't the primary admin user I can delete them. So I think yes - although I doubt it has been tried often or is worth it. If you delete all the transactions you probably mess up your books (assuming the transactions were valid to begin with).  There are many things in QuickBooks (like accounts, vendors, employees) that can't be deleted but can be made inactive. We really need an "inactive" mode for users, too.