What is the best way to deal with cranks?
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!