C# and SQL – How to Handle or Prevent All Admins and Users Being Removed

The only way I know of to build a truly secure web site is to require a password and have a challenge/response device.

For example, TreasuryDirect.gov sends you a card with a grid of random characters. As part of the logon process, you are asked to provide the characters that are contained in specific cells of the grid. But this is expensive and time-consuming; it requires to get the cards, to track them and to send them out. Plus, if a person loses the card, they can't log in until you get them a new card. In the case of TreasuryDirect, where you could be managing millions of dollars, this hassle is worth it. Only you can determine if it will be worth it for your site.

Another option is to add a validation picture, like a lot of banks do. I believe this option only helps thwart some phishing attacks so if you think phishing will be a problem, this might be an option.

I believe the other alternatives, including what most banks use, seem to be secure but aren't really. Security questions, for example, I believe compromise sites more than they help. Others think this too.

One other thing will be to encourage users to use a "Passphrase" instead of a password as Jeff Atwood recommends. Amazon has taken this approach as well with their newish PayPhrase.

There is no security reason to separate the login information from the rest of the user information. It doesn't make your site safer in any meaningful way, so you should just organize the tables by what is most logical.

The real security issue in this domain is that you should not be storing plain text passwords. Only store a salted hash of the password.

That being said, it probably makes logical sense to separate the user information into a few tables, because you have different users with different roles. Have a user table with all the common information, then customer, employee, etc.