Can ballots abstain?

Well, each ballot cast in the prisci subreferendum is either for or against the main referendum.

Rulebook 6.3.2.2 will read:

Polling. All Methuselahs may now cast any votes and ballots they have (see below) at this time, in any order. They call out their votes and ballots freely, and there is no obligation to cast votes or ballots. Once a vote or a ballot has been cast, however, it cannot be changed. The polling stage is completed only when all Methuselahs are finished casting votes and ballots(if you need a time limit, the players can agree to wait 15 seconds after the last vote or ballot cast to close the polling stage).