Insults, character assassination campaigns and politicizing the existence of these bioweapons would be a good way to do that. Just copy paste some of the comments here and change the name to insult anyone who thinks they exist. They could then go and kill millions and not receive any retaliation whatsoever with people praising them for their effective program of keeping the disease epidemic they created under control. Even if you got the guy who discovered AIDS and won the Nobel prize for it to say that these were gain of function viruses that incorporated HIV protein parts, you could just launch a big propaganda campaign to attack his character.[1] Much cheaper than having to fight a war.
[1]https://www.gmanetwork.com/news/scitech/science/736458/frenc...