Super Diplomat? Bribe other races to attack the race which possesses Beyond Mortality, and cut any alliance you may have with them. Selling them junk technology usually buys a war and gets you some money at the same time, although if tech trading is off, 1500-2500 bc should do the trick. You can even join in yourself and help in bringing down the offenders, or you could maintain peaceful relations with them and even bribe them into declaring war on others(esp. Super Organizers/Manipulators).

If possible though, the simplest option is just to go and kill them yourself, though having a few "allies" in the mix may help speed things up. Just declaring war seems to make the AI shift over to military production and put research/social behind. Offensive espionage is generally not amazingly effective.

Looks like you were doomed anyway

You could try to get an alliance with that race, especially since they were already allied with many others, and finish off all non-allied races. Though that might be tough to do in 20 weeks.

They had already won. Via an effective Diplomacy Victory - which in turn allowed them to get an official technology victory.

Yah, pretty much... If a race totally outclasses you militarily and is allied with every other race on the map, you had lost already and just weren't accepting it until they got the technology victory.

That kind of situation is not only an effective Diplomacy victory, but a gauranteed military victory too if they had wanted to pursue it. I mean, it seriously sounds like they had you heavily outmatched in terms of military, clearly were very advanced technologically, and had every base covered diplomatically. It's what I would call a good old fashioned butt-kicking on all fronts, really.