Gigi and KatieRabbit have given some great ideas.

For my part, I would recommend the following:
1) I don't think you can control what your DS plays with when outside your home. So let his grandparents buy what they want for their house. He won't be there that much to be overly exposed to things you don't want him to be, right?
2) Mention in casual conversation to your firends about the type of things you want for your DS and the type you don't.
3) If people you are not that close to and not comfortable being blunt with want to buy him toys - let them. As you don't know these people so well, chances they won't know that your DS will not be playing with the toys. You can then either regift them, or make them the toys that you take to the beach/the park/on holiday, i.e. things that you won't mind if they get broken or lost...