RationalWiki defines a crank as:
someone who holds extremely unorthodox views on a subject and is often very vocal about these opinions. A crank will usually maintain their viewpoint despite, or perhaps because of, evidence to the contrary. The crank is usually an amateur in the field they are arguing against, but sometimes individuals with expertise in that field will become a ‘crank’
It is our policy to ban people we believe are cranks whenever they start to comment on this site. Even if such person does not violate any particular point of our comment policy. The reason that we nevertheless ban such people is that we do not expect that a constructive discussion is possible with them. And in order to protect others from wasting their time by debating those persons, we ban them. Nor do we want to provide them a platform.
A decision to ban some one by reason being a crank, is always taken with care. We will, if possible, do an internet search on that person and if applicable visit their [personal] websites.
Our experience with commenters has given us a certain intuition which commenters are likely to be cranks. And we know that there is no use in debating those people.