There is a very easy way to prevent Aliens Colonial Marines from happening, as Kerethos laid out above. Read a review of the game before buying it. But you are right, at E3 it's not the completed game and it's wrong to place your faith in it, it's still publicity and a lot could change.
In that context, what E3 does is give you suggestions as to which games you might want to look out for and research upon release. It's true there's still a lot of hype and marketing there, but people are physically playing an (unfinished) version of the game and that's the best evidence you have to go on before the game is actually released and you can start reading reviews.
For example, I was very iffy about Shadows of Mordor. The plot looked like fanfic. The action looked like it was literally torn out of Assassins Creed. As in they'd actually taken the animations for Assassins Creed and put them directly into the game without thinking about what that meant in terms of character. Finally the Nemesis system sounded like hokey rubbish that would boil down to the wonders of the Find the Blue/Red/Green/Purple/Indigio Key random quests. But the previews have been much more positive and suggested that the nemesis system may be more flexible and interesting than that. This doesn't mean I'm about to pre-order the game, because I don't want an Aliens Colonial Marine, but it does mean I'm going to listen out for it's release and read reviews about it when it does release.