Android 7.0 call block feature
I do something very similar on my device running Marshmallow to prevent tele callers and quite successfully so :)

automation is the preferred option. Many apps are available. I prefer using MacroDroid , since it is free (upto 5 macros) and easy to learn. 

Since I don't have Nougat can't test, but should run fine

In the first macro, it is configured here for non contacts. You can select individually the numbers in your case




The SMS portion of first macro is expanded below - you can configure it to your liking



Note: Two macros are strictly not required. You can add action of second macro in first itself - I needed two macros for customisation which is not shown here