I'm not sure why people would blame Spike for anything here.
Spike's company would hire an advertising firm to create posters designs and ads for the film. Juan did contract work for the ad agency. When you do such work, you sign documents giving the agency full rights to any work you have done for them. He says he was paid a small sum and he did the work hoping to get some recognition. This is exactly how contract work is done for ad agencies. If anyone misled the artist, it was the agency and not Spike, but if the guy was paid anything, it's the end of the story. It didn't work out for him and now that he's griping, I suggest other ad agencies will not want to hire the guy because the agency isn't looking to create trouble for the film producer and that's just what the artist has done
certainly, Spike can be a horse's ass, but in this case, he is squeaky clean