In CTP player can ask each other to refrain from spying/pirating activity. I think this should be introduced in BtS. Each time a spy's caught causing trouble (stealing tech, poisoning water, etc) there will be a -2 diplo penalty. If the spy is caught while there is such a treaty in place, the penalty will be doubled to -4, or give the offended player the choice to declare war, in addition, there will be a -1 or -2 penalty with every civ (for showing allegedly "bad character"). AI should be programmed to largely respect such agreement, but when they have a strategic plan to do certain things, they would disregard this agreement. This should work like Right of Passage in civ3, if a civ is caught violating spying agreement for a few times.