I've seen and experienced this kind of behavior. My wife had to clean out the house of a hoarder, a good friend of hers, when the house was condemned by the city, then manage her affairs. Comparable to the worst of the houses I've ever seen on Hoarders, no working kitchen, only running water a hose in the back yard, etc.
What I've seen is these people have a very strong desire to accumulate stuff, and a very strong attachment to their stuff, so strong that the only way they can part with it is to give it to someone they also have a strong attachment to. So, loved ones, don't expect them to end this habit any time soon, the upside is that it means they love you. They may not express it the way you would like, but they do. There is, I think, also an element of "I shouldn't buy anything more for myself, but it is ok to buy it as a gift for someone else.", my wife has some of this, though fortunately is also ruthless about cleaning things out, especially after the experience with her friend.
Since you can't change them, you can only change your reaction to it. My suggestion, which we've used with some friends who were in the habit of giving us stuff we didn't want, was to make a deal with them. Any gifts we gave each other that weren't things specifically requested, the receiver could do whatever they wanted with them. Regift, donate, etc. It softens the blow somewhat if you make this a 2-way deal, even if you don't ever plan to give them useless crap like they do you. :) Then you can donate guilt-free, and remind them of the deal if they ask you what happened to that amazing avocado peeler.