How often are they around to witness their gifts being opened?
While my mom has never sent anything I explicitly said not to (I never bothered telling her those things, since it would ensure we'd get them), she is kind of a mild hoarder who with every packages sends all kinds of weird old stuff (from unopened paper cocktail napkin packages from the 1970s to old candy bought on sale the previous year to ugly old outfits from me as a kid that I refused to wear, ect.). Talking to her about it would be useless and would only get into a huge dramatic fight (which upsets me and excites her so I try to avoid those). Instead, as annoying and grumbling as it made me, I just made a policy of opening everything at the garbage can/recyling bin. Salvagable/unopened stuff that I think would NOT be a burden to Goodwill (don't send ancient TJ Maxx soap-and-hand lotion sets that are so old even the price tag has yellowed and cracked, it costs them a ton of money to dispose of the garbage people bring to them) I put directly in my trunk in a box and when the box is full I bring it to the store.
Peace and harmony reign. I can "release" the resentment by not even having it enter my house. My mom can release some of her pack rat stuff by sending her garbage to me. I don't need to feel that I have to take that junk into my house and clutter it up. I'm never going to be able to prevent her from sending ancient stickers from the 70s (so old they don't even stick to the paper anymore) "for the kids". So I just deal with it on my end.
My parents live a continent away though, so it's easier than if they lived in the same town. But, when they've visited and mom drags out her junk (there's less of it when they visit because my dad prevents her from packing the suitcases with it I think), I smile and nod, and it all goes away after the holidays (they go overboard on everything, so the kids never get attached to yellowed formal gloves or squishy balls with imprints from real estate title companies, in the post holiday shuffle).
You do need to be prepared for some awkward situations. My mother once flipped her lid because she saw a really ugly shawl (in my most hated color, which she knows) she gave me in the dress up box (it was soft, DD loved it) because she said it cost $70. I just said "oops, sorry", took it out, and when she left I put it back in the dress up box. She asked about some keychains (more logo giveaways from her business) and if the kids liked them, and I said that they didn't really play with them. She did buy DD a barbie and a barbie kitchen playset (for the barbies, not kid kid sized) when I was in my "Barbie is the Devil" phase, for Christmas when they were visiting--I had to bite my tongue, but really my DD lost interested after grandma left because really only grandma wanted to play with it, and later on I passed it on to a friend whose daughters loved barbies who didn't have a problem with them. Keeping my cool got easier every time.
So I guess, before you have some Big Discussion, you really need to be honest with yourself. Do you think there is any chance of the inlaws changing their behavior? If so, then by all means, have a kind discussion with them. Wouldn't hurt to ask them not to get that stuff, right? But if you know in your heart of hearts that their behavior isn't going to change, then don't waste your time trying to get them to do anything. Spend your time coming up with an action plan, and getting yourself ready to carry it out with no fuss and no drama. :)