Rude dissent.

At a scrum, the referee [penalises the Pumas. Having done so, he blows his whistle and calls the captain over.

The referee says to the captain: "I don't know which of the two of them but I'm not going to tolerate such language to a match official.

"Talk to your players about abuse, please."

Within the referee's power?

Law 10.4 (s) All players must respect the authority of the referee. They must not dispute the referee’s decisions. They must stop playing at once when the referee blows the whistle except at a kick-off or at a penalty kick following admonishment, temporary suspension, or send-off.
Sanction: Penalty kick

It is within the referee's power all right and disrespect of the referee's authority falls under foul play, even when it is not in foul language.

Anything else the referee could have done?

If he had known who the culprit was, he could have sent him from the field, to the sin bin or permanently off. In this case he did not know who the culprit was but he could still have marched the Pumas on another 10 metres.

Law 10 deals with Foul Play. The law  regards dissent as foul play.