I think what you really mean to ask is, "How do we prevent people from asking questions whose answers will be quickly made obsolete?"
The answer to this question is, don't even try. Instead, embrace this class of question on the condition that it will be treated in a way that is compatible with one of the most important principles behind the Stack Exchange format--specifically, the notion that a "good" answer is one that provides value to future visitors of the site.
Before I suggest a simple yet somewhat radical solution, let me present one likely future for this site: a year after launch, nobody will care what the best unlocked Verizon LTE smartphone was today. After 2 years, people will start to become annoyed at the site because it's a cluttered mess. Even before that happens, people will start to systematically duplicate every question whose answers have been rendered obsolete, for the guaranteed rep boosts. Allowing dozens or hundreds of practically duplicate sets questions over time, most of which have obsolete answers, will only serve to clutter up the site and bury any current information, making the "related" sidebars and simple searches on the site practically useless. If we follow one of the other suggested solutions for this problem, it will be impossible to find certain types of current hardware recommendations without specifying an appropriate date tag. After 5 years, the site will be about just as worthless for finding current hardware recommendations as the forums of yesteryear. The problem is not with the questions, and not with the answers; but rather, the fact that (a) many answers effectively have expiration dates after which they will have no value to future visitors, and (b) the failure to deal with this fact appropriately.
I mentioned earlier that I have a simple but somewhat radical solution. Brace yourself, because here's the radical part: Any answer with a known or unknown expiration date has no value future visitors after that date, and should be deleted once it is rendered obsolete.
"But wait!!!!!" you say, "If you delete all those answers, that's going to wreak havoc on everyone's reps on this site!" And that brings me to the simple part: don't give them the rep in the first place...nothing gained, so nothing to lose.
That said, if nobody is a fan of converting to a CW immediately, no problem. Just wait until after the first review to convert to a CW, or don't convert it. Rep for any "worthwhile" post (score of 3 or greater and visible for at least 60 days) is preserved even after deletion, including rep gained from a question or its answers prior to that question's conversion to a community wiki.
So the 2-part solution is this: for any question whose answers are guaranteed to be ephemeral, require that the question be made a community wiki. Cull obsolete answers periodically.
The process would go more or less as follows:
- Mod sees an otherwise acceptable question, but realizes its answers will be obsolete within a short period of time (e.g., 6-12 months).
- Mod tags the question with the year and month at which it should be next reviewed (e.g., review-2015-12 for "review again in December 2015") and optionally converts the question to a community wiki.
- Each month, mods and high-rep users review the questions that were tagged with that month.
- If there is a new answer which now makes an older answer obsolete, the old answer is deleted.
- The old "next review" tag is deleted and a new one is added for some point in the future.
Because the question is a community wiki, all answers are automatically community wikis. Again, after the question is converted, no rep is gained for an upvoted answer, and no rep is lost when the obsolete answers are deleted.
(Note: see meta for clarification on what happens to rep if an answer was posted and upvoted before the question--and consequently answer--was converted to a community wiki.)