Take the current highest-yield nuclear bomb and destroy England right before the begin of their collonial era.
Generally speaking, I believe removing a global superpower just before they do their world-changing thing is probably going to have the biggest effect on the timeline.
Everyone was building empires at the time and fighting over who got what. All that nuking England would do is to mean France, the netherlands, germany, spain etc would get more bits
France wasn't a pushover around WW2. They had enough manpower to fight nazi Germany toe to toe. What they did, however, was underestimate how fast they could advance. France also ignored a warning that the germans were amassing to push through the Ardennes, which allowed the nazis to face little resistance on that front. Apparently, if they took immediate action, they could've mobilized an air raid to completely destroy that nazi battalion, which would royally fuck up the rest of Hitler's plans
It would only be a deterrent for empire building if there's a pattern (probably 3 or 4 similar events), otherwise people would consider it a random hateful act of god, of which there are many, and of which have been interpreted many different ways
I was focussing on changing the timeline, not on deterring nations from doing something. Without English colonializers, there would have certainly been other colonizers, but e.g. the whole China situation would have likely been very different. There would not be a dominant anglo culture right now. No English-speaking USA, no English-speaking Australia, no large countries with an anglo common law-based legal system.