This feature is available in Enterprise and Ultimate editions.
Teams Feature is ON
If a member is no longer working on a team, you can remove that member's row on the Capacity Planning page using the Remove action. Removing a member also removes any capacity attributed to that member in open sprints, but retains entered capacity for closed sprints. The row can always be recalled to view historical entries by selecting the Show All Contributing Members filter setting. If the member rejoins the team, you can add them back to the list.
Teams Feature is OFF
Capacity is specified per project, per member, per sprint. The member list on the Capacity Planning page shows all members assigned to the project plus anyone who has capacity assigned to them in the project, so there is no explicit Remove action at the member level. You can always remove capacity entries for any member going forward if they are no longer working on the project.