Prevent users from starting multiple accounts?

Question

I know that in the end it, can\'t be done.

But, what are the options to:

  a) limit the options for persons to create multiple accounts,
  b)

Answer 1

The most difficult to break methods I've seen implemented in real life are to use a separate hardware medium for confirmation (sending a confirmation code via SMS for a public service, or mailing an RSA token for something more sensitive, like intranet access), or to ask for a financially-bound piece of identification, for example a bank account number (Paypal deposits a few cents to your account and the sum of the amounts is your passcode) or a valid credit card number.