There's very little to stop such forking from taking place. There are international treaties in place between many countries to recognize IP rights granted by other treaty participants, but I'm sure there are countries that are not signatories to such treaties (generally they are poorer countries that see being a signatory as being a disadvantage, something I think they are probably right about).

In such a case, where the fork is being operated in one or more countries that don't recognize the IP rights, the countries that abide by those IP treaties could enact laws preventing their residents from connecting to and using such networks, but they wouldn't legally be able to stop the entire network from operating.

Also, it's worth noting that if they (e.g. the US) did pass legislation of this sort, it would be very difficult to enforce, and probably drive such currencies to a black market.