Hi Sue,
I'm on V18, I changed the "Automatically update membership" options under the Rule Sets tab for the audiences used to control enrolment for a particular certification.
Not sure if it will work differently for Programs, but so long as that particular audience isn't used for anything else, it should be fine to freeze membership into the audience. It does mean you can't use that audience for anything else though.
https://totara.help/docs/audience-rules#automatically-update-membership