If, prior to the casting of absentee ballots, it is made to appear by due proof to the county auditor or election board that any voter who has marked and forwarded an absentee ballot is no longer eligible to vote due to death, felony conviction, or mental incompetence prior to the opening of the polls on the date of the election, the ballot of the voter must be returned in the sealed return envelope with the evidence of the disqualifying factor listed in this section attached and the envelope marked accordingly with one of the following statements:
The marked envelope must be returned to the officer in charge of the conduct of the election. A returned absentee ballot deemed ineligible may not be opened or counted. The casting of any such ballot shall not invalidate the election.
SDCL 12-19-9.2