SE isn't going to remove items purchased and used on an account. You could later claim you changed your mind and you wanted the items back again, or that you never wanted the items removed and someone was impersonating you.
Delete the items that can be deleted and stop using the ones you can't if it bugs you that much.
Or when asked, he simply states that after buying all those mounts he had a change of heart and doesn't support the cash shop anymore. As long as he does not purchase any additional items in the future, there shouldn't be a problem.
People change their personal values and beliefs over time. There's no point in taking someone to task for something trivial they did when they were younger if their values have since changed and they are no longer doing that thing.
