The technical problem here is the fact that if you block one IP, you're probably not blocking just one site, you're blocking several, or even hundreds or thousands, many of which maybe have nothing to do with gambling.
There is also the question of whether these types of measures do more harm than good in terms of suppressing free speech, guaranteed by the First Amendment. This kind of content filtering at the ISP level contributes to the fragmentation of the Internet, making us all less and less connected. Furthermore, if courts continue to force ISPs - which are usually private corporations - to police the Internet, it might turn into a very ugly place where your private data is monitored and the choice of information you can obtain is severely limited. Yes, forcing ISPs to block gambling sites will probably stop most people in Minnesota from gambling online, but how much overall damage will it do?