What is the best way to deal with cranks? Imagine you are minding your own business when you receive an email from someone who claims to have discovered the Answer to the Ultimate Question of Life, the Universe, and Everything. He provides some &quot;details&quot; about his theory, then says he wants to meet you and / or arrange a time for a phone conservation to discuss it. What should you do?
The only thing I've seen is this 1983 article by Underwood Dudley about what to do when the trisector comes. Dudley says not to examine the trisector's proof, and also not to direct the trisector to the proof that trisection is impossible. Instead one should send computer-generated results showing that the trisection is imperfect. Problem with this is that it only works for questions that can be attacked by computers - there was a recent question here on Academia SE challenging conservation of angular momentum which this won't work for.
What are some general guidelines for dealing with these situations?
Edit: ignoring an email is easy, but what if the crank calls or visits in person?
Ignore the crank.

Unless you are in some location where you are required by law to respond to communications (I remember reading here about one country where it was indeed the case that it was required to answer emails as part of "open records" laws), you are under no obligation to waste your time on a crank's musings.

If it amuses you to respond, feel free to do so, but you waste your time at your own peril!