I'd like my neighbor to stop moving units through my territory, but I can't seem to find an option to cancel our Open Borders agreement. In the deal history screen, I can see an Open Borders agreement from well over 30 turns ago that doesn't even show up under Active Deals. The trade screen just says that we already have an Open Borders agreement. Am I missing something, or is there no way for me to cancel this agreement?
Agreements can't simply be canceled, that's why they're agreements for a certain number of turns. Think contracts. You have to decide if you're able and willing to handle the terms of the agreement (contract) before committing to it. If you break the agreement, it has serious retributions (or rather, you have to go to extreme lengths, war, and face the backlash to break an agreement). If that wasn't so there would be little point to most agreements.
"Yeah, let's do this defensive pact thing, unless of course you're being attacked, at which point I may simply decide to back out of the agreement..."
Update (2010/10/14): Apparently your problem is a bug and will be fixed.
- Diplomacy – Fix for never ending deals (peace, research agreements, etc).