What percentage of users do this, that you can tell? If its 3% of users, it might not even be worth it.

That being said, why not allow a few registrations per IP address (versus 1). You'd hate to punish possible paying clients by preventing them from signing up for your service because someone else on the same IP did the same thing.

Finally, it may be worth it to send an email to the people you notice and ask how you can improve the product. It would let them know that you know what they are doing, but you still want them as paying customers, not to punish them or push them away.