The best method to remove an S2D cluster entirely is reinstalling Windows Server (or AzureStack HCI) from scratch.
Storage Spaces Direct is a deeply integrated essential part of the Windows Server operating system that touches nearly every aspect of it. That is why the best option to create a clean cluster is to install the operating system fresh and wipe the disks during the process.
Cleaning up your Active Directory, even after a proper cluster demotion, is also crucial. There may still be lots of remnants left that may need manual removal.
- Start a new Windows Server installation.
- Run diskpart during the installation wizard and perform disk clean for each disk in the server.
- Finish the installation.
- Install all the updates.
- Create a new S2D cluster (using a different name and IP is recommended).
If you are up to build a hyper-converged solution, you may also be interested in replacing Storage Spaces Direct with Virtual SAN https://www.starwindsoftware.com/vsan. While having a similar feature set it does not require ReFS and offers much better performance and storage efficiency for smaller clusters.