I'm not certain, if the AI takes the Diplomacy-ability into account when trading amongst itself, but I would guess so. Therefore, removing all Diplomacy-techs from this race would be my first choice, because a low Diplo-ability will make all trades much more expensive. You could also further reduce their Diplo-ability by editing their .raceconfigxml. Just open the file in a text editor, search for Diplomacy, and put in a negative number.
Another possibility is to reduce the WillingnessToTrade value of all techs for this race. However, this will be a lot of work. Plus, there is no guarantee it will work, because there are several more things that factor into how willing the AI is to trade something (AIValue, type of tech, current relations, militarisation, etc.).