I wish to extend the deadline for applicants who have already begun their application but do not wish for new applicants to create an application.
Typically, the waive feature would serve best in this case. However, if there are 50+ applicants, this can be work-intensive.
A possible solution:
If the program is not active (thus not visible on program search) but the date record is active (deadline is extended to a future date) for the program, the existing applicants should be fine to proceed forward with the application and the advisor can avoid new applications.
Note that this could be a problem if you are recruiting for future terms for the same program.