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 :(.