How to prevent unwanted calls and SMS on an android 8 phone without having to install any application?
The first thing is create a contact in which we will store the numbers that we want to block (yes, I will use the original name “SPAM” for the contact). Then we will add the numbers we want to block to the contact: 
Contact SPAM
Once the contact is created we edit it, and in the hamburger menu, we will select the option “Block contact”:
Contact hamburger menu
One the conctact is blocked we won’t receive any calls or messages for these numbers from now on: 
Contact block question dialog
Enjoy! ;)