Preventing Duplicate Leads Based on IP Address

ip addy probably won't work for you, as most business browsers are browsing from an office where all users share a network external ip.  So unless you want to limit trials to one user per department or building, that's probably a no-go.
I think I'd use that email addy, and send them something at that addy in the flow to confirm the trial, which would at least limit folks to a number of trials no greater than the number of email addys they actually receive.

I would suggest that you send confirmation link via a trigger campaign which says if lead has filled out form, send thank you email with url to trial. And in your trigger campaign option you put, "run once" which means that email address has to be valid and there is only one trial per email. Of course this won't prevent 1 person to use multiple emails...

Trevor, what you're describing sounds like cost of doing business with trials initiated online. I suspect that whatever you might put in place to deter the people from using different emails will also deter some legitimate customers from doing the trial at all. If you want to make it easy to trial, you should be willing to accept some level of abuse. Those guys probably wouldn't have been good customers anyway. Thoughts?

Thanks everyone for these comments. We are a B2C business (unlike most MKTO customers), so single corporate IP's are almost never an issue. It is unfortunate, but people use temporary emails just to get another trial, so we are just trying to do a better job policing using what MKTO provides. 

If we setup the campaign to only allow leads through once and it is tracked at the cookie level (regardless of email changes, etc.) then am I to suppose the would not be issued another trial through the form since they are the same "cookie"? If so that is great unless they delete all cookies :(.