I Killed the Drengin. I captured all of their planets. I killed all of their ships. For many turns they have been asking for peace each and every turn. (They still have 3 rally points.) They were gone from the diplomacy screen until I reloaded the game (I took a break for the night -- saved the game, shut down my PC went to bed -- then booted, started the game and loaded from the save file). They are still asking for peace and have returned to the diplomacy screen. When I "speak to" them the "Lord Kona" screen shows they have no planets, no ships, no shipyards, no resources, nothing but some left over credits and three techs I have not yet researched. I was able to negotiate a trade for those three techs and for peace. --- With a dead race?
"Lord Kona" screen is at https://www.dropbox.com/s/twfn6gdhzz65nhe/Screenshot%202014-12-29%2017.17.02%20Dead%20Drengin%20Diplomacy.png?dl=0
I created an SDSupportTool zip file and opened a ticket with the zip file and the save game file.