I have an enemy who is crossing my land to trade with another civ. Can I stop them from crossing without going to war?
No, you cannot prevent another nation's traders from crossing your territory.
Once you research the Early Empire civic, you gain the ability to enforce borders against military units and allow friendly nations to cross them.
However, traders and religious units are exempt from closed borders, and can wander freely into your territory. See Civ VI Border Rules for more information.
Trade routes can navigate to their destination, regardless of the border, as long as you have a free trading capacity, a trader unit, and a valid destination.
Short answer: No you cannot prevent traders passing through without going to war.
- You have discovered the city
- You are not at war with the civ it belongs to
- Within the trading range of origin city (extendable by trading posts)
- A route is available (you've explored enough passable tiles between the locations)
You can only plunder trade routes when you are at war with another civilization. Barbarians can also plunder the routes if they're left unguarded.
Note: If you go to war with another civilization, all routes between you two will be canceled. If those routes going through your lands are not to your own cities, they will persist, allowing you to destroy them after declaring war.