Too many gifts and inappropriate gifts are problems too many of us have to deal with. Yes, it does sound a little ungrateful to complain about people giving our kids too many presents or saying that the presents are somehow not right for our kids, but when people force things on you that don't align with your values it causes a bunch of stress.
There are about a billion approaches you can take to dealing with gifts you don't want (for your kids and for yourself), but I think the issues you need to take into consideration are:
1. Will the giver know what happened to the gift after they gave it to you?
2. Is it important to you (for whatever reason) that the giver understand your position on gifts and abide by it, or do you just want to avoid the gifts?
3. What kind of relationship are you trying to create with the giver?
If you can clarify these issues, you'll know how to proceed. For example, a friend of mine who lives across the country from her MIL, who visits only very rarely (so she won't know what happens to the gifts), but with whom she has a very cordial relationship (that she'd like to keep happy), routinely takes absolutely everything her MIL sends them as gifts straight to the Salvation Army. To my friend, it's absolutely not important to clarify her position on the gifts her MIL gives. Her MIL buys things because she likes them, and enjoys giving them, but doesn't care what happens to them afterward. My friend gives them away and doesn't have to keep them in her house. Everyone wins.
But not everyone is so lucky. Some people have givers who insist upon seeing their inappropriate gifts in use. Some of us find it important to delineate our boundaries about gifts that we feel are and aren't appropriate (no licensed characters, no toy guns, no plastic toys, no organic items, etc.) so we engage bewildered grandparents and aunts and uncles in discussions and hope they remember from year to year that we'd rather have gift certificates to independent booksellers instead of W*lmart. Some of us really just don't want anything more to deal with in our already over-stimulating living spaces, but relatives think giving more and more presents shows love.
So it's really important to figure out exactly what you can achieve in terms of getting the kinds of gifts you want (or very few gifts) while also maintaining the best relationship you can with each giver. From there, proceed to either a discussion or some way to avoid the problem. One thing many families do is either decide to buy each other experiences (like trips to museums, or special grandma-grandson dinners, or a 3-day trip for everyone in the family to go on together) so they can avoid the stuff problem but keep the closeness, or draw names so each person only buys for one other person. If you can't agree on a blanket solution for your family like that, however, you might have to deal with each giver on a case-by-case basis.
A personal anecdote: My uncle, for whom my younger son is named, emailed me to ask what the boys wanted for Christmas. I answered that they didn't want anything, but if he wanted to give something they'd be happy with money for their college funds. (I'm sure this is horribly gauche, but he's my only uncle on that side and we're close in an I'll-tease-you-until-you-bleed kind of way.) He countered by saying that boys need presents, so he was getting them each a puppy. I raised him to two puppies apiece, and he countered by agreeing on a few Thomas engines for both to share. I don't really want more Thomas engines, and he'd rather give them an indoor trampoline featuring an airhorn, but it was a win-win because we both stayed in character about it and respected each other's respective needs.
One last note: A blogger I admire told me her surefire idea, which is that she has a special big Rubbermaid storage container especially for things that she's going to regift. She puts a Post-It on each gift with the name of the person who gave it to her, so she never gives back anything to the person who gave it to her. Just brilliant.