In my altarian game I've cornered the dregin to 1 planet. I destroyed their only shipyard and offered peace for all of their technology, ships, and 4000bc per turn. The dregin are now -50000bc in debt and climbing while I'm making 4000bc per turn off of this single planet so I've surrounded the planet and am protecting it from being invaded by another race. The drengin are essentially out of the game because their crippling debt prevents them from building and I am now absurdly rich.
Update: I basically destroyed the game with this. After taking the last drengin planet the treaty continued giving me money infinitely despite their race being gone. I tried this with the krynn and the yor and the results were the same. Eventually i reached 2,000,000bc and suddenly it swithced to -2,000,000bc causing my production and research shut down. Also the destroyed civilizations will send me random messages asking for peace and once they reappeared in the united planets votes but notably had 0% of the vote.