After the death of Sirius, Kreacher was sent to Hogwarts by Harry.
suggestion, you could send him to Hogwarts to work in the kitchen there. In that way, the other house-elves could keep an eye on him.”
“Yeah,” said Harry in relief, “yeah, I’ll do that."
However, some of what he stole was stolen before this:
Or perhaps Mundungus, who had pilfered plenty from this house both before
and after Sirius died?
It seems the locket was among this from what Kreacher said:
How do you know Mundungus Fletcher stole the locket?”
“Kreacher saw him!” gasped the elf as tears poured over his snout and into his mouth full of graying teeth. “Kreacher saw him coming out of Kreacher’s cupboard with his hands full of Kreacher’s treasures. Kreacher told the sneak thief to stop, but Mundungus Fletcher laughed and r-ran. . . .”
As for why he didn't use magic on him, it's unclear. My best guess is that he was forbidden to hurt members of the Order by Sirius (or Harry, for that matter.)