Yes, you can do this any time you like.
From Your External Bank App
Although each bank app works slightly differently it should be easy to do.
Within your banking app, or online account, try to find the list of active open banking connections. If this is difficult, simply do an internet search (i.e. google) for "remove open banking consent" plus the name of your bank.
Every bank allows you to revoke your consents. Note, Marygold is likely to appear under 'Moneyhub' in the list of active consents.