Remove diplomatic status for anyone involved in such activities, put oversight on the diplomacy, make a big international public stink out of it (this matters), share intel with other governments and create a working body to root out this, implement local programs so that schools/companies etc. are aware that this happens and should report it, consider other forms of action.

But all of that would be pretty hard in the current geopolitical situation, there would be a huge price to pay.

Australia was slapped with huge trade war in the order of billions for pointing out that China was lying about COVID origins.