That's exactly what it is. The hosting country can ONLY just tell them they are "persona non grata". (Expel them.)
Of course, they may face charges in their home country or be extradited. Their home country can waive their diplomatic immunity - and do so for their diplomats acting criminally in other countries.
But 100% of all of these actions are on the home country. The host country can do absolutely nothing besides expel them or complain to their home country.
Let me know if you see anything to the contrary here or anywhere else online: