I can tell you for GCII. In the tech definitions file (techtree.xml for the base, then one for each race that are mostly tweaks), for each tech there is an attribute <WillingnessToTrade> with a nominal range of {0..100}. There is also a <No Trade> flag that I don't recall the proper name of. I rather suspect that GCIII is doing something quite similar.
First figure out how to mod, and where to put the changed files. Then find the tech trees and do some reading. It should be easy to dope out a fix.
Gosh, I hope it's not totally different in GCIII...