I would like it if trade goods were more like trade.
What I mean is, I would like to option of leasing trade goods for a finite amount of time, i.e. making a deal to lease nanorecords for the next two years in exchange for a lump some of money, payments, technology, or whatever else you can trade. If you go to war with the supplier, you lose access to that particular good, until your able to make peace and cut a new deal or conquer him completely and steal it away from him.
In the game trade goods are like wonders, things so unique that they can't be copied or reverse engineered. It would reason to believe that if a alien race if providing you with something that makes your ships faster or tougher,helped you use diplomacy to gather allies against them, sped up you research, or made your people happy and healthy that they would stop as soon as you declared war on them.
If you wanted to get really complex you could have setup so only the ships built during the leasing period get the bonuses, all other trade good effects would just effect your civilizations statistics.
In the games current state, the best thing to do is try to be the one that manufactures all trade goods at all cost and never trade them away to anyone other than a minor race. If you let someone else discover them first, you can still get the bonuses through trade, but so can everyone else. Getting access to the trade good form some one else doesn't give you an advantage it only makes you even.
In a recent game I traded a research and pop growth trade good to a friendly major race in hope to get them to like me enough to form an alliance and some quick cash. I had a large tech and pop advantage and didn't see the harm. In less than a year my "advantages" has disappeared. My game basically ended when my "friendly neighbors" got Overlords while I was still trying to research avatars and decided they wanted to expand their empire and my expense.