Analysis of the logs from one of my sites showed that IP addresses were re-used much more frequently than I expected them to be 
you can hope that they are on a fixed IP at home or at work, but they can always use an ip spoofer/anonymiser if they want to.
It doesn't matter what you do, they'll always be able to re-register, unless you want the inconvenience of verifying every user yourself, and even then they'll slip through. Does your site have any kind of community around, or in, it? If so, peer pressure is often the best way to deal with people like this, particularly if there's an 'ignore this person' feature in the forum.
As for roles, you can always set up a role the everyone is automatically subscribed to, and have that role be the one that is permitted to all your functions, then just remove awkward customers from that role. You could also add them to a 'banned customer' role so that you can keep track of them.
Depressing, isn't it? People like this are a royal pain!