I'd be very wary about requiring users to use their PayPal account. This would imply that you were going to be taking money from users at some point. This would put even more off.
The best solution is probably the simplest. Do what Stack Exchange does. Allow the user to sign up with OpenID (or even just e-mail and password) and then track the IP addresses the user logs in from and who the user gives reputation to and who they get it from. There are probably other things you can track such as the time of day they usually log in etc.
Then have tools for your site moderators (you'll need moderators) to show users that share IP addresses and have interacted with each other a little too much (for example). Depending on what you track you can show different data.
Obviously one drawback with the IP address is that it's not unique. People at the same workplace will share one as will people using the site from internet cafes or libraries.
Now on Stack Exchange where the post is more important that the person colleagues shouldn't be voting for each other more than they would for any random person on the site. However, if on your site the person is more important then the reverse could well be true.
What ever system you implement people will try to circumvent it, so this will be an ongoing process of changing sign-in rules etc.