Very recently I made a huge miscalculation during my D&D campaign.

The players in my D&D campaign were chasing after several pieces of a broken magical item. They had been spending their time researching in the capital and were slowly finding more info about these pieces.

They eventually found out that one piece was being protected by an adult green dragon, so naturally this team of 4 level 6 adventurers went on to take it from the dragon.

What I had planned was a stealth encounter where they would have to sneak by the dragon slowly, while trying to get closer to his huge gold pile where the fragment was, and possibly reward them with a minor magic item.

However, they decided to rush forward and kill the dragon for reasons unbeknownst to me. They somehow succeeded; nat 20 after nat 20 they kept destroying the dragon and eventually succeeding. So naturally they took his entire gold pile of gems and magic items back to the capital and have now gotten themselves a huge amount of other magic items and have blown the balancing system completely out of the water.

Encounters that should be around their difficulty have suddenly become way too easy, and puzzles and problems have become even easier with all of their new magical items. I know if this continues they will become either bored or they will become power obsessed murder-hobos.

I have no idea how to actually balance this back without striping them of their magic items completely (which I know will piss them off). How can I correct this mistake and balance things back out? What can I do to get things back on track?