Removing volunteers is a fairly simply process, similar to most areas of the system.
To remove a volunteer:
Click Volunteers, then select Volunteers.
Search for the contact record you wish to modify.
Click More, then click Delete.
If there's nothing blocking the Delete action, such as the volunteer record being actively linked to an opportunity, the system will remove the volunteer record.
This will also clear the Volunteer checkbox found under the main contact, in the Details tab.