It was their first time playing ttrpgs, but still.

The point of murderhoboing is to create chaos. Don't give them what they want, just forbid murderhoboing entirely. If there is a non-murderhobo in the party, that person doesn't deserve to get wiped and if the party is all murderhobos, you should find a different party.

Set bounties on them. Put up wanted posters in major cities. Now they can no longer find important NPCs and they are constantly attacked by adventuring parties and mercenaries.