I just got off the phone with XBox support to cancel my Groove subscription. A few notes:
1) I was the only person this person has ever heard from to cancel their Groove Music pass, clearly either a super loyal or just obscure fan base
2) I cancelled within 30 days of my credit card being billed so I received a full refund
3) The credit card I paid for my Music pass with was somehow not on file, so I received a microsoft credit for 120% of the value
It may required a phone call, but 20% extra if the credit card isn’t available.