How can I prevent users from creating multiple accounts?

Question

I'm building a site that needs to guarantee user reputation scores are accurate by preventing users from creating more than one account, at the cost of decreased user signups. So far, the only solutions I have thought of are allowing users to:

• Link to their (verified) PayPal account through PayPal Account Authentication
• Provide their PGP public key, and checking that the MSD (a metric of trustworthiness) is below a certain value

Of course, even these methods aren't bulletproof, but are likely to make creating a sock-puppet account very difficult. Are there any others I haven't considered?

Answer 1

Charge them real money for each account.

Answer 2

From this StackOverflow question:

• Place cookies on the users computer
• Log their IP
• Store content into their localStorage (only for users with HTML5 browsers)
• If you really want to start digging deeper, you can start putting restrictions based on the users session length, how many pages they navigated prior to voting, i.e. starting to profile the users that try to circumvent the system, and start putting restrictions on those profiled users.
• MEMORY tables on server with ip addresses
• evercookie
• browser fingerprinting
• required registration
• http://code.google.com/p/mailvalidator/
• make list of banned domains

Answer 3

I think there are varying degrees of bulletproof. So other ones might include: - Linking to a Facebook account - Google account - other Oauth providers

It's a tradeoff between making it so hard to get an ordinary account when you aren't trying to game the system vs the ease of those who want to creating duplicate Facebook profiles/PayPal accounts.