My Drop-Shipping Shopify Store is based in India where Cash on Delivery (Manual Payment) is the most commonly used method of payment for online shopping. My problem is, when somebody uses this form of method, offering an option of choosing a different billing address is like an invitation for fraud.
I want to deactivate "Use a different billing address" option when/if somebody chooses CoD (Manual Payment) as their preferred mode of payment.
First off, I see that you are using the manual payment method for COD. I'd suggest you used Shopify's own Advanced Cash on Delivery app for dealing with COD orders in India. It has some great features built in (like limiting pin codes for COD, max order value etc.)
About deactivating the Billing Address, this is not possible to do that. However, something that works for everyone is enabling the "Use the shipping address as the billing address by default" option. It can be seen in your Shopify Admin -> Settings -> Checkout -> Order processing.
To deal with COD fraud and RTO's I'd recommend you check CODfirm out (I lead the team there so let me know if you need any help with COD setup in the contact us on the app).
Hope this helps.