This entire event was a staged press op. Keith Alexander is a ~30 year veteran of SIGINT, electronic warfare, and intelligence, and a Four-Star US Army General --- which is a bigger deal than you probably think it is. He's a spy chief in the truest sense
and a master politician. Anyone who thinks he walked into that conference hall in Caesars without a near perfect forecast of the outcome of the speech is kidding themselves.
Heckling Alexander played right into the strategy. It gave him an opportunity to look reasonable compared to his detractors, and, more generally (and alarmingly), to have the NSA look more reasonable compared to opponents of NSA surveillance. It allowed him to "split the vote" with audience reactions, getting people who probably have serious misgivings about NSA programs to applaud his calm and graceful handling of shouted insults; many of those people probably applauded simply to protest the hecklers, who after all were making it harder for them to follow what Alexander was trying to say.
There was no serious Q&A on offer at the keynote. The questions were pre-screened; all attendees could do was vote on them. There was no possibility that anything would come of this speech other than an effectively unchallenged full-throated defense of the NSA's programs.
Even the premise of the keynote was calculated to wrong-foot NSA opponents. However much you might want to hear Alexander account for the activities of the NSA, the NSA itself is not the real oversight mechanism for the NSA! My guess is that no pol with meaningful oversight over NSA would have consented to address a room full of technology professionals about NSA's programs; they were happy to send NSA's own supremely well-trained figurehead to do that for them.
I think a walkout might have been effective, had it been organized well enough in advance (perhaps with some of the same aplomb as the [I think misguided] opposition to CISPA); at least you'd get some stinging photos.