Disable COD when my own carrier service is chosen.

[ssodano]

Hi all, i'm new on Shopify.
I'm trying to build public app for give to final user a possibility to do delivery with my company through carrier service.
The problem is that my company not handle shipping with cash on delivery. So, i'm looking for disable cash on delivery (if is enabled on the store who install our app).
The problem occurred because the final user, first select shipping method and only after he can choose if pay with credit card or cash on delivery. Can you suggest some solution ?