You can send them a letter to the permanent partner processing centre in Melbourne. I would also email them to be on the safe side too.
I found this for the 100 about the information you need to include but you can include the same information for the 801: https://www.border.gov.au/visas/supp...onsorship.aspx
I just want to say it is nice to see someone doing the right thing and withdrawing sponsorship when the relationship is over. We see a lot on this forum that don't withdraw the sponsorship when the relationship is over.