You can declare war on a rival civilization. During a war, an ally cannot defect to the enemy's side, even if the enemy's influence becomes greater as a result of some factors. For example, the influence was almost equal: 150 for us and 140 for the enemy. We promised protection, but the enemy did not. During the war, CS was damaged and the rate of decline of our influence is now 3 times higher. After peace is concluded, the former enemy can become a new ally on the very next turn, since his influence may be greater.
If CSs have created a lot of units (10+), then you can declare wars as a preventive measure, so that these units are killed and in the future you don’t have to fight with them. Although with Authority you will want to kill these units yourself for the sake of culture and science.
Losing allies and gaining new ones is a normal mechanic of the game - all empires receive quests and are able to produce emissaries. Build two or three forts and roads next to the CS, deploy 1-2 units if the CS is no longer an ally.
Also, war with an enemy CS is a good opportunity to capture that CS without the opinion penalty of other CSs (the minimum influence will be -20 instead of 0 after the second declaration of war on any CS) for resources, like an airfield or a buffer city protecting the core of your empire.
Or you can set up a citadel during a war to capture someone else's embassy and then set up your own embassy during a truce, gaining an additional vote in Congress. The installation of a citadel during war does not cause negative opinion.
