How do I stop taking Cash on Delivery and accept only prepaid orders?

Go to Settings → Store Settings → Payment Gateway and untick Cash on Delivery in the Enabled column of the Payment Methods table. You'll see "Cash on Delivery disabled" and COD disappears from checkout.

Before you do this, make sure at least one other method is switched on: an online gateway, Bank Transfer or UPI. If nothing is enabled, shoppers see "No payment methods are currently available. Please contact the store." and can't place an order.

To bring COD back later, tick the box again.