The game will make anyone take participation in a war when they hit level 12.
There is no avoiding that since it’s considered to be tutorial.
After that war ends, the person may choose not to participate in wars by clicking the option in the Alliance menu (tick box).
Sadly I found no way around that.
The best thing which I found is how to make a defender’s team illegal for war, thus the player has to be a spectator.
The game, however, still counts him as an active war participant, resulting in the opponent’s team having 1 more additional team.
I had an issue with my solo alliance when 10 of my side accounts hit level 12 almost simultaneously. I made their defense teams illegal for the war, and we ended up with 2 of us vs 12 of them. Needless to say, we stood no chance.
How to make a participant’s defense team illegal for war? Simple - register a team, then eat (train with) anyone on that team. Say, your team contains a 2* feeder hero Needler. Just eat it after you register the team and there you have it! Illegal defense team.