Prevent users from starting multiple accounts?

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?

I'm assuming you're talking about a free service? I can't think of any ways that don't either have serious drawbacks or would be trivial to defeat. Things like setting a cookie, requiring a unique e-mail address are easy to defeat.

Requiring a unique IP address is not foolproof but might work to some degree, up to the point that you have lots of users and get complaints from people behind proxies.

The best ways are to charge money or require people provide some kind of personal information, like real name/phone/address that you verify, or a CC number, but that's invasive (then again maybe you only want serious users who are willing to provide this sort of info).

I guess I would turn the question around and ask "Why don't you want to let people have multiple accounts?"

There may be some other ways of mitigating whatever your underlying reason is, i.e. if you're worried about lots of orphaned blogs you could scan for a period of inactivity and disable them or at least schedule them to be looked at by a human. If you're worried about spam blogs you could periodically scan all blog content for spammy stuff. If you're worried about bots and are using some generic software like WordPress, change the names of the form variables and otherwise protect your forms from bots.

Definitely think of other ways of dealing with the problem, because you are not going to be able to block people from registering multiple accounts if it's a typical free service like Blogger.

As for detecting multiple accounts by one person, the first thing you need to do is have a log file store complete data on every user login (username, timestamp, IP, user-agent etc.), that you can then analyze later. I'll list a few things to look out for, but just by poring over the log file you will likely discover other patterns. Some ideas of things to look for are:

• Set a tracking cookie (i.e. random hash) and log its value on login, look for multiple logins from the same cookie value
• Logins from same IP address/user-agent combination
• Logins from same IP address only (less reliable than the previous two bullets)
• Accounts with email addresses from free webmail services (Gmail etc.)
• Accounts with same password

If you're worried about spam blogs, you could try doing some analysis of blog content, i.e. extract all the <a href>s and look for correlations between blogs. You could run the blog content itself though something like SpamAssassin or otherwise filter for spammy words like "viagra" and "rolex."