If you get a fair number of pitches for crank theories, you may want the following close reason (we use it on the Physics site)
We deal with mainstream mathematics here. Posts on topics don't belong in reputable journals is not appropriate on this site.
(you may want to say "mainstream research level mathematics", too)
@Kaveh suggested this one:
Questions about the general correctness of unpublished preprints and solutions of major open problems are off-topic. For more information, see [our policy for questions about preprints].
However, it only applies to questions that ask for correctness. It may be necessary for the close reason to cover posts about crank theories as well.
The best word that describes such posts is "crank", It's accurate, and quite unambiguous. However, it's quite rude and unprofessional to use the term, for obvious reasons. I feel that giving an objective definition to mainstream may work:
We deal with mainstream mathematics here. Posts should be about topics that have been published in reputable journals. Questions about the general correctness of unpublished preprints and solutions of major open problems are off-topic. For more information, see [our policy for questions about preprints].
Questions about X are off-topic because Y. For more information, see [Z](link to meta post).
Feel free to change it, but it can be helpful to have as a starting point. $\endgroup$for historical reasons
implies that similar questions are not welcome anymore. $\endgroup$