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.
- Look on the schedule to see if it says "repeating" next to their name
- Look in the preassignments pane to look for their name. By deleting the preassignment, they will no longer be placed on future schedules.
- 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.