Did the corporation at one point properly elect to be an S Corp?  In other words, in the past, did the corp properly file 1120S?  

An S corp's may elect to revoke its S Status.  Formal filing of the revocation is required.  See   https://www.irs.gov/forms-pubs/revoking-a-subchapter-s-election

The S corp status is automatically terminated if something, which would prohibit the corporation from making the election to begin with, has occurred.  E.g. a second class of stock; number of shareholders over the limit, a corporation owns a share of the S corp..  any relative of E.T., Superman (from Krypton) or Dick Solomon (from the unknown planet on the Cepheus-Draco border) bought some shares without living on earth for 183 days.... etc.

Creating a situation that bars the corp to be an S corp would be the way to go without filing the revocation.  There are filing due date considerations (based on the effective date of the revocation) and other consequences.