This issue is still not solved and I now have Dreamhost support staff helping out.

They've unsubscribed on the server, deleted the mailbox on the server, I've deleted it on my client, gone and edited config files on the client to remove subscriptions and all mentioned I could find in any pref file, unsubscribed from webmail and combinations of all of these. Now I've asked the staff to find out if they can tell how the folders are being created in the first place to try and find out which end is responsible.