It is entirely legitimate to down-vote completely crackpot ideas

Would it be legitimate for me to downvote people the next time they mention ideas that I consider completely crackpot (like quantum immortality), or ideas that most of the world considers completely crackpot (like many-worlds)?

The arguments contained in the ancestor post criticize "Here Be Dragons" for actual failings contained therein. That the specific ideas mentioned (9/11 conspiracy, autism-vaccine connection) are considered crackpot doesn't mean one is allowed to strawman them. It's all the more reason that one doesn''t even have to strawman them.

“Not at all. It is entirely legitimate to down-vote completely crackpot ideas purely because hearing the same old completely crackpot ideas can be annoying.”

Just because you call something crackpot doesn’t mean its true. Just because you call something legitimate doesn’t mean its true.

Indeed you are committing a circular argument fallacy right now.

“Circular arguments are arguments that assume what they’re trying to prove. If the conclusion of an argument is also one of its reasons, then the argument is circular.”

http://www.criticalthinking.org.uk/unit2/fundamentals/logicalfallacies/circularity/

And by criticising me instead of my argument, you are also committing an ad hominem fallacy.

““Ad hominem” is Latin for “against the man”. The ad hominem fallacy is the fallacy of attacking the person offering an argument rather than the argument itself.”

http://www.criticalthinking.org.uk/unit2/fundamentals/logicalfallacies/adhominem/

The only problem is. Every time you fail to PROVE it is crackpot. You commit another one of these fallacies.

"entirely different kind of 'conspiracy' used in law"

There is absolutely NO DIFFERENCE between the conspiracies used in law.

Your dangerously close to making an appeal to authority fallacy. These 3 usually come as a set.

“An appeal to an authority is an argument that attempts to establish its conclusion by citing a perceived authority who claims that the conclusion is true. In all cases, appeals to authority are fallacious; no matter how well-respected someone is, it is possible for them to make a mistake.”

http://www.criticalthinking.org.uk/unit2/fundamentals/logicalfallacies/appealtoauthority/