You can’t control their behavior. You can only control your own. With that in mind, here’s how I handled the situation in my family (also no kids):
I told them the best gift they could give me was to permit me to “drop out” of the whole holiday debacle, that although I appreciate their intentions, their gifts simply leave me with the dilemma of keeping something I don’t want or need, v. giving away something cherished solely on the basis that they gave it to me.
I told them that if they simply couldn’t resist giving me something, please make it a consumable: A nice candle, bottle of wine, cookies, preserves or candy, something like that. And that’s all I give them. Every year I send boxes of favored goodies that are homemade and consumable. Either given or received, if it’s something unwanted it can always be passed along to someone else.
Lastly, I told them that if they disregarded my wish, I would consider it a gesture of disrespect. Which frankly, it is. Don’t ask unless you want the honest answer!
It took them a couple years to accept I was completely serious, but now they do and have for years. In fact, they expressed appreciation that to no longer have to run around town trying to find something for the person who, like you, just goes and buys what she needs when she needs it.
I do try to do things for them during the holidays that contribute to special memories, even if it’s not something I would ordinarily do. I’ll go to Midnight Mass with my stepmom though I’m not religious, make a special pie for my dad even if I don’t eat it. And if I do happen across something I am certain is a gift they would love, I send it – at any time of year.
Good luck. These old family traditions are fraught with peril when we choose to change them.