This is awful but sadly to be expected. The Smurfs IP is something they are milking the cash cow of still today with the movies and whatnot, and I am sorry it happened to you hipoonios and that this has pushed you to not wanting to work on Amiga stuff anymore.
This is just another example of how fucked up things are with assholes who own IPs like this throwing lawsuits left and right. As an opposite, we have the positive example of Nintendo who only recently has said that they would allow fans to create fan projects using their characters, names and IP. This is a great precedent but I see very few companies following their steps.
I mean, what the FUCK can Peyo do with 2000 euros, when they make a shit ton of money with the movies and whatnot? This is just insulting, going after a small guy and trying to con money out of them. Pathetic and disgusting. My guess is, too, that this is actually the law firm at work, trying to justify the hefty monthly sum Peyo must pay them by going for the throat of every little bitty bullshit they found online mentioning the Smurfs or whatever. Highly doubt Peyo took notice of your game or actually gives much of a shit, but they also can't let one through or they set precedence that they let one through and that doesn't help them for further, probably more valuable lawsuits.
I recommend you now to stop posting about this online, it can only play against you, and get some good lawyer who might be able to work on contingency and help you settle this out on your favor. Hopefully you have good news for us soon enough. I agree with Mrs. Beanbag that this is unorthodox and you shouldn't pay before getting legal counseling. This looks like a settlement but you didn't even meet them. As she says, a sue comes followed by a letter from a court. Don't pay unless a lawyer tells you to.