Is there anything we can do to prevent the multiple "how can we keep Trump out of the White House" variants? Can we post a reminder to search prior questions before posting?
"Can we do anything" - Yes.
If the question is asking the same thing that was already answered in another question, close it as a duplicate (if you don't have required reputation to VTC, comment and flag for closing)
- For extra credit, expand the duplicate target with a new answer or an edit to existing good answer, to turn it into what SE considers "canonical post".
If the question is unanswerable (any answer is subjective opinion, or just a rant), close it with appropriate close reason (same caveat re: not enough rep).
If a question is trivial and can be answered with simple Google search, or Wiki lookup, or otherwise bad, downvote it (and copy Google query into a comment explaining why you downvoted, for extra credit).
If the question has a good idea, but a ranty/ConspTheory component, you can salvage it by editing out the crud and leave the good question inside. Alternately, simply post a better more objective version of a question if it's too unsalvageable, though this approach has ethical implications depending on how much of an idea you borrow.
If you simply are tired of seeing the questions on a specific topic, assuming they are tagged properly, you can ignore specific tags on a site. Or, y'know, just not pay attention. And do what Londoners are advised to do if they don't like the weather.