When in diplomacy, if a civ you have good relations with wants a tech or resource but you don't want to give it to them, and at the same time, you don't want to have a '-1' put against your name, then do the following:
1. Demand appears on your screen.
2. Select to negotiate the deal
3. Once the screen has changed, choose to EXIT the negotiations with no hit on your reputation.
What do we consider this? An exploit? I dont do this all the time in single player however there have been times where a close friend wants iron (or another critical resource) and I dont want to give it to them. I use this to get out of the negotiations.