This can be done by simply downgrading. Go to your "Billing" page, "Overview", press Change plan:

Pressing "Change Plan" will take you to "Select your plan" page:

You should go ahead and click the "Downgrate" button under the "Free" membership.
This is how you stop fund withdrawals from your card.

Please note:  if you have purchased additional disc space in the past, it is necessary to unsubscribe from the “Disk” service separately by clicking the “Unsubscribe” button in the “Disk space” menu of your personal account.

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