I wanted to know about how can I ban certain selected customers from selecting Cash on Delivery Payment Method?

You can not ban specific customers from placing COD orders. However, StoreHippo provides you option to allow/disallow COD for specific products or pincodes.

To have specific products available for COD, please use Product based COD app 

To restrict customers from placing COD orders only when they are from specific pincodes, use Delivery Pincodes app.