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. 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 Cheap Omega Watches 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. extract all the 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."
You can't and you shouldn't. You are not dealing with the real world guys, but with accounts, so treat them as abstract entities which have the equal rights to live.
Some options I can imagine on the Replica Omega fly:
-- Only one account for email address. But I can create more then one email. or use Mailinator.
-- Long and tedious verification procedure. But that will discourage the users from registration
-- bind the IP to the account and block(temporarily?) that IP from creation of another account. But two different users with the same gateway will be blocked.
-- Use the cookies. But the user can delete them.
If you had it to your Terms of Service, you could make it enforceable and delete user's accounts if they have more than one. Although as seen by Digg, this is not a popular option.
ROLEX INTRODUCTION Part 1