The obvious answer would be "They should listen to critics, and avoid the film!" But that's a dicey thing when it comes to popcorn fare, because a lot of it doesn't receive stellar reviews from critics, but it's still an enjoyable, pulpy ride. I don't subscribe to the idea of turning one's brain off once you buy your ticket, but there's something to be said for watching a movie like Crank or Punisher: War Zone once in a while.
Audiences also don't know when they should listen to critics -- or who they should listen to. Sure, general audiences should find it in their mouse-fingers to seek out Rotten Tomatoes but a lot of people hear only the soundbites of Ben Lyons (or their local equivalent), or see the blandly 'It's stupid, but you'll probably like it" reviews in their newspapers. I know a lot of otherwise intelligent people who go see a bad summer flick because they'd read a good review in the newspaper or heard it on the radio.
Plus, we all know there's a pretty respectable list of films that were savaged by critics upon release, but became celebrated and admired after collecting a bit of dust ... and they're held up as examples of smart, independent fandom winning out over staid, critical institutions. Think for yourself becomes the manta, especially for an edgy or amusing underdog.
Now, I'm not saying that there's ever much chance of a Wolverine or a Terminator: Salvation* being an underdog in disguise. I just don't know that it's entirely fair to sneer at audiences as "stupid" when a crappy film is a financial success. Perhaps I bristle at it because I still pay to see them. I do it not only because its my job to be part of the wider discussion, but because I constantly hang onto the hope that "they" could be wrong about it. I know a lot of intelligent moviegoers who do the same thing, and unfortunately we all contribute to the studio's gleeful success. But it isn't audiences who make a bad movie ... we need to put the blame for that where it belongs.
*I still haven't seen this, I'm basing that purely on the angry reviews I've seen.