This being the realm of Law, there of course is a Latin term for the action of refusing/expelling a diplomat: persona non grata.
A diplomat declared PNG may legally be confined to their embassy/consulate until they leave the country at the earliest opportunity, and both of the directly involved countries have the option to speed up that process by chartering a flight to a neutral third country.
Serbia being involved, Australian authorities could take the opportunity to deal with a related matter at the same time and make the third country the Netherlands, specifically Rotterdam-Den Haag airport so a few other Serbs could be put on the flight and dropped off at the International Criminal Tribunal for the former Yugoslavia.