To cancel a "Request Sub" that has already been submitted by one of your volunteers, open MSP, go to the Schedules pane, and open the schedule containing the swap requested date. Once the schedule is open, locate the service and position for which the minister requested a sub. You will see the words "Swap Requested" to the right of their name. Right click on this and a menu will appear. Select "Clear Current Flag."
This will remove the "Swap Requested" flag. Once the Live schedule is saved and closed, the Web Terminal will be updated and the request sub action will be undone.
Volunteers also have the option to cancel sub requests from the "My Schedule" tab when they are logged in online.