I got rid of my land line a few years ago, but even when I had it, I never used it for anything other than making calls to places I didnt want to get my cell phone number (lol).
The company I had my land line through (verizon, at the time) had a web portal where you could perform some small administration actions. It had a feature where you could set your landline to only ring for certain phone numbers... up to 10 I think it was.
I set it up so only family members phone numbers actually rang on the land line, and and any other calls were discarded. Solved the spam call problem on land line for me, but I didnt give that number to anyone but family, so anyone that had it was a spammer, thus why I set it up that way.
Check into whoever provides your land line to see if they have some online management capabilities, or consider getting rid of it and porting the number to a cell line, where there is more functionality on blocking all calls except people on your contact lists, etc.