"Sub only" volunteer got scheduled
There is likely a preassignment or team that this volunteer is a part of. These assignments override "sub" status and place the volunteer in the fixed rotation.
1. Look on the schedule to see if it says "Repeating" next to their name
2. Look in the Preassignments pane to look for their name. By deleting the preassignment, they will no longer be placed on future schedules.
3. If no preassignment exists, go to the Teams pane and look to see if they are on a team. Edit the team and remove them.