How many such requests sets do you need to end-date ? Might be quicker using the GUI
There is no API as such to end date request set. You have to right custom program to update the tables to end date and at the same time before end-date make sure if those request sets scheduled are also cancelled.
FND_SET is the API for request set which has the procedure DELETE_SET to delete the request set.
There is no procedure in the above API related to end_date.