I actually contacted them with this very query. They told me I can post my card back to them to ‘Freepost Monzo Recycling’ and they’ll get get rid off it from there.
You need to send it back if you cancel, according to the app.
Not sure if this is optional, as I already have a second metal card (First was replaced by Monzo as a fraud risk), and I was never told to return the first.