Tell them exactly what you DO want. We had this with my mother. She had always been very "possession orientated" and when I lived at home, birthdays and christmas was all about who'd spent most and who's piles of presents were biggest. It was fine when you're a child, but as an adult, it was shockingly ridiculous.
I nipped it in the bud when we got married. Didn't wait for the first christmas or birthday, but I got the message sent out long beforehand that it was time to stop and that we should just buy each-other token or personal gifts. I made the first move by asking my mother what she wanted and told her it was basically going to be one "main" gift and then I'd make it up with something else such as a toiletry set or box of chocolates. Thankfully, she got the message and we got into a habit of telling eachother what we wanted.
So when our son was born, the seed had already been set. Beforehand, she wanted to know what to buy (amazing as a few years earlier sh'ed have just gone out to buy a load of stuff). We had a conversation and the outcome was that she would buy the cot. To prevent any mistakes, we made a day of it, and bought it together, at the same time, we bought the pram, and a load of other stuff. That open dialogue thankfully continued and we could openly discuss what our son needed and what mother would like to buy. Of course, she still bought "surprises" but they were a lot smaller and cheaper, things like small clothing items and small toys/books, etc. But she felt better having been part of the buying of the big stuff.
I think you've missed the boat if it's already happening. You should have nipped it in the bud long ago. But it's still a conversation worth trying. You don't have to be nasty, or make a big deal out of it, but how about at the next birthday or christmas, you politely suggest that they buy something substantial instead of lots of tat. But, handing something back isn't acceptable I'm afraid, it's bolting the stable door after the horse had bolted. You need to smile and be grateful short term and come up with a plan to stop it being repeated.