If limiting users to one account were so easy, there wouldn’t be sploggers and fake profiles out there… So short answer is, can’t be done.
Long answer, you could put something in place to record the IP of the user signing up and check if that IP address has signed up before, but it wouldn’t be completely reliable, the user could easily access site via a proxy to bypass the check…
Longer answer, maybe don’t even be too concerned, it’s gonna be rare that a user will have that much time or content to be using multiple free accounts like this.
Instead, you might want to look for plugins that protect against signup SPAM (aka “splogging”) to reduce this kind of thing happening from the start.