Even without a license, there still is intellectual property, and as the guys live in Spain, they should know that. Here the legal owner of the art would be Continuum (or the other art creators), he is the one entitled to denounce the abusive use.
The best course of action would be to send send them an email explaining to them that the art they are using ain't supposed to be used yet, asking them to remove it until Mutant Rising is released, and then to credit the authors.
if they don't comply, you should send a second letter threatening of legal action, even if I doubt the art creators are willing to launch an international procedure against them.
But, you still could ask the hosting provider to remove the content of their game, since it use illegal content (you just have to prove it's yours with some screeshots). And of course the nma community could start a spoiling campaign of their work...
But I don't think it will come to that, since they don't really speak english, they probably didn't care to make the difference between whats free of use and what isn't, a friendly warning letter reminding them of their obligations should do the trick.
But doing nothing and ignoring them is fine too.
