I agree that keeping superheroes and such away from your DS isn't really going to make a difference. Sorry, but little boys especially, love to play rough and will be action figure like characters while pretending whether or not they are exposed to them. They'll make guns out of anything - even their own hand will suffice - so while I get where you are coming from, it's likely that your extended family continues to try to give these gifts b/c they know little boys like them, and it's not going to be the one thing that makes your DS play a bit violently.
I understand your concerns, I really do. DS was always prone to nightmares, and we had to be especially careful with what he watched on tv. When he was 4 and getting ready to go into preschool I didn't want him to be the odd kid out and the only one not knowing about superheros, so we did a gradual exposure. Yeah, totally addicted, with little sis close behind. One thing to look into, either for the child you watch or yours when he's older are the "Classic" superhero videos. These are the OLD Superman and Justice League cartoons that are extremely mild in violence levels. As for the superhero movies with actual people in them, I can't really think of any we've ok'd for DS to watch yet (he's 8) simply because of the violence and language. He has seen the first couple of Star Wars that were released, the 3 Muskateers (LOVED it!) and the first and third Indiana Jones, but that's only been within the past year. If it weren't for the whole masterbation discussion and the fact I'm really not ready to answer questions on that just yet if I don't have to, I'd consider letting him watch Transformers.