Until air defense gets fixed, players can ignore virtually all aggression. Full stop. The war game - the only part of the game where you really do interact with other players, is broken currently.
If we ignore that, we can also say that boycotts are basically useless. Nuke bans are basically useless. Security council measures besides providing aid to new players is pretty dang useless. Players targeted by sanctions can simply sell nukes or supplies to themselves from another country or from their enterprise to completely bypass any sanctions. Additionally, federations are not useful since there is almost no war, so there are almost never situations where you could rally enough players to get the minimum 10 votes for anything anyways. There's probably only one federation in the entire game that can use the security council for sanctions.
Regarding war levels, their existence *creates* doubt and fear in player minds. It tells players they could or should be scared, which usually is enough to deter players from ever moving up much in level and facing the "scary unknown" of war level 3.
Players are taught by the game to be afraid and they almost never recover from that. You're right that a "war world" exists, but NO NEW PLAYER SHOULD EVER SIGN UP IMMEDIATELY FOR OPEN PVP IN ANY GAME. No one competes against top players out of the gate in sports games, in shooter games, or in other strategy games. This isn't a good solution.
Furthermore, almost no one sticks around long enough to "graduate," or progress to the war world. You need to practice and accumulate assets before you're ready to fight other players, but players simply just don't do that. It took me a year to venture onto FB back before there were war levels at all, and at times, I have been one of the most aggressive and warlike players around. An average or "normal" player will never ever consider it, even though they might end up liking it, and it's partially because of how war levels are presented and how they play out.
As a result, what happens most often is a new player chooses a non-war world, gets scared of going up in war levels, can't find anyone to help them learn the game and quits. We have the short-term single player mode going on right now more often than I've ever seen it before.
Lastly, there are players whose primary motivation appears to be simply to irritate others. There is NO POSSIBLE WAY to interact with them if they choose to stay (or get stuck) at low war levels, so they can exist in perpetuity as an annoyance to most of the community. There are no repercussions. They're toxic, unhelpful, and selfish, and players who wish to teach them a lesson or "compete" against them have no options for doing so.
There's just so little benefit or call to player interaction these days that the social side of the game is basically gone.