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) increase the chance of detecting multiple accounts / person

for a blog-like web service?
(people can sign up for their own blog)

Update:
I think the 'limit the options' has been answered nicely. (there is no reliable method, but we can raise the bar)
However, I would still like to know what other options there are to detect multiple accounts?

Answer 1

I think, as many people have mentioned above, one of the best ways is to verify mobile numbers and that's what I wanted to use in teh first place, if it wasnt so damn expensive ...

I have found this site here and I think it can be used for this purpose but I havent tested it myself, but it seems pretty modern and cheap

Answer 2

Ask users to register with a credit card. You don't have to charge anything to the card, you can just check that the card is valid.

Answer 3

You can set a cross browser cookie e.g. http://samy.pl/evercookie/ (flash cookies) The can not be deleted by the browser cookie deletetion they stay 4 ever and you can read the cookie cross browsers. Its the ultimate solution if the user uses the same computer. With more than 1 computer the IP address is your only way to find out, but (in my case) sometimes 2 real people in the same house with 2 computers login 2 my website

Answer 4

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.