My mil used to do that, i looked for the reasons, she is a widow, gets a bit lonely and likes to 'give' something so i tried to alter it to make it more useful
She used to buy cheap/nasty clothes for dcs both here and abroad, i now tell her exactly what would be useful and am gushingly grateful if she can find it, i always offer to pay for it all/some of the cost, she accepts for pricier things and covers the cost of cheaper things but gets the joy out of 'giving' to the dc regardless
she would make jumpers and clothes for dc which i hated, so i went shopping with her found material that i liked, explained my prefernces (colourfast/hardwearing cloth, real wool blend not acrylic) and she has now made things i like, the dc wear and are practical
there is an m and s discount store near her, she would buy 'bargains' which werent helpful (she never fully got that if buying a size or two up you needed to change season) so i started asking for things i would like, so i now say to her could she check if the have baby vests in such a size (i could get these from anywhere tesco for eg) but then she will go and pick and feel useful and it cost me the same £ wise
i just say no to certain house tat but do allow things the kids will like (though prob not use)
she also does it with food which i am finding harder as i hate to waste food but am learning that actually an unwanted gift of a past its best apple wont be the end of the world i will just bin it and think of it as not really MY wastage
hth