Open the Phone app
Tap the clock icon, or select Recent Calls
Find the offending number
Tap on the offending number, but not near the phone or message icons
{a menu should appear}
Select Block Number
Answer the following question in the affirmative
Now, to be fair, this is how to block a number.
Unfortunately, it is often not really that easy. {It looks harder above than it really is.}
That is why there are apps to take care of this sort of thing.
Sure, Googe Play Store has its share of apps to do this {or at least claim to}, but I prefer open source apps for a number of reasons.
Simplicity and lack of ads are a big two of them.
If the above solution is not adequate, try BlackList Blocker.
It is what I use.
https://f-droid.org/en/packages/com.kaliturin.blacklist/