I don't want my subscription to renew automatically, I want to disable auto-renewal.

It is not possible to disable auto-renewal. However, you can schedule a cancellation anytime that will take place at the end of the contract.
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You can cancel your subscription by going to Organization dropdown (top left) > Organization Settings > Subscription > Manage Subscription > Cancel Subscription (bottom right). This will cancel the subscription and downgrade the organization to a free Developer plan at the end of the current billing cycle. With this, you can effectively avoid auto-renewal.