As said you could eliminate thier trade routes but that wont prevent anyone from trading with them just prevent them from trading with others
as for talking to them once you uave U.T. You can talk to anyone so i dont think you can prevent people from talking to them.
Now then in one of the dev streams i asked about races haveing custom +/- diplomacy values and got a "yeah you should be able to do that " kind of response. If thats the case you should be able to modify them to just hate everything
you're trading with us we hate that; war
you're not trading with us. War
you're touching my borders. War
you're to far awya from us. War