I had a bad feeling when I first saw your game - wish I'd said something at the time.
Smurfs is pretty hot property at the moment (last major film in 2013). I bet no one would have bothered you if The Smurfs franchise hadn't had a revival in the past few years.
Hope you find some resolution to it all. I'd ignore the demands for money - I would hazard a guess they'll probably leave you alone now you've removed the 'offending' material.