Well, it could be done from the database, but the problem is how to do it without deleting all of the users, especially your own admin user. The safest way is to do it from the UI.

If there is only one user that you want to keep and you want to delete the rest of them, you could find out the userid of the one you want to keep and then delete all others with sql like this:

DELETE FROM mp_UserProperties
WHERE UserGuid IN (SELECT UserGuid FROM mp_Users WHERE SiteID = @SiteID AND UserID <> @MyUserID)

DELETE FROM mp_UserRoles
WHERE UserID IN (SELECT UserID FROM mp_Users WHERE SiteID = @SiteID)
AND UserID <> @MyUserID

DELETE FROM mp_UserLocation
WHERE UserGuid IN (SELECT UserGuid FROM mp_Users WHERE SiteID = @SiteID)
AND UserID <> @MyUserID

DELETE FROM mp_UserPages WHERE UserGuid IN (SELECT UserGuid FROM mp_Users WHERE SiteID = @SiteID AND UserID <> @MyUserID)

DELETE FROM mp_Users WHERE SiteID = @SiteID
AND UserID <> @MyUserID

Best to backup your db before attempting anything like that.

Hope it helps,

Joe