MY PLAYERS ARE TOO POWERFUL!

so I DM with a group of 6 players, all level 6, their class and Species don't matter but what does is that they KILL EVERYTHING!

I put them up against 3 goblins protecting a relic and bam they're all dead, 2 boars scrounging for food? Dead, a bandit holding a woman hostage? Dead

Like I don't know what to do I've told my players that if they don't stop killing things too fast I'm ending the campaign, is there any other tips as to how I can make a challenge like more of a harder rating or something?

Edit 1: I heard the term "murder hobos" thrown around in r/rpghorrorstories, so they must be murder hobos then