I think about how I would react as a player.
I know I would be upset if you took one of my planets, by any means, but we are talking about influence.
The first time, I would complain at you. If we are friends, I would forgive you, but I would get resentful.
The second time, I would chew you out with emphasis. I might respond diplomatically or economically if I can.
The third time, I would flat out warn you of military consequences.
Fourth time, war is declared. I'm not putting up with it anymore. For one thing, you are probably starting to win the game and I have do something about that.
I figure the AI should follow some behavior like that with variations according to flavor. For some, step three is optional. I hope that diplomatic/economic alternatives will be available. I hope that some sort of cultural response will be available as well, so that flipping isn't completely inevitable. As a mostly peaceful player, I find it fun to play with influence effects. Having the AI competently compete in that field should be great fun.