I wonder what expelling advisors and banning embassies will do.
I'm not so sure myself, but I think banning embassies could stop another nation from initiating diplomatic actions in that country for a time.
Along that line expelling advisors could probably be the option that makes it harder for another nation to influnce the country.
You can reduce their influence in a country; make it harder for them to gain influence for a while, even reduce the country’s opinion of them.