I imagine that if economic embargoes/sanctions were to be enforced over the internet, it wouldn't be enforced at the website level. Think about how impractical it would be to require every webmaster out there to keep an updated list of embargoed countries and have to block all visitors from that country.
And it's not like a North Korean internet user has a direct line to your servers. How many networks sit between you and the North Korean internet user? If the ISPs responsible for those networks are allowed to deliver those packets from North Korea to you, and from you to North Korea, then how can you be in the wrong for serving data for those ISPs to forward to North Korea?
To give an analogy, if I can make calls to and receive calls from Cuba, that means the telecom provider I use is actively providing a connecting service from here to Cuba. If anyone were to be prosecuted for the phone call, it would be them before me.
Frankly, if the government doesn't want you doing business with a particular country or entity, they'll make it really hard to do so. It's not likely something you can do inadvertently. Chances are, if you're not allowed to do business with a particular country, then your payment processor isn't allowed to either. If you're not allowed to send packets to a country, then your web host and their ISP isn't allowed to either.
