DA:O with full DLC and expansions, and tell them not to blow the world up until they finish it on the hardest difficulty, with completing every side quest. That should give us a ton of time to either:
a) evacuate
b) place tactical nukes inside their motherships
c) send a commando team and take control of one of their motherships
(Seriously, that game took forever to finish. I loved it, but it was extremely long)
Failing that, I'd show them Duke Nukem Forever and tell them that this is what happened to the last aliens that tried to wipe us out.
Failing even that, I'd have them play Bioshock and, when they finished ask the 'Would you kindly leave us alone?'