Get a cordless telephone that can block the calls (mine blocks up to 250 numbers), or get a call-blocking device that you install on your phone line (those can probably block a thousand phone numbers). On a cellphone, call blocking might be built-in or you can download apps to do it.
Also, look for a unit that can block the 'first ring' of a blocked call, so you never even hear the phone ring when it blocks a caller.
Depending on your phone, bring up the number in your logs, go to settings, and a drop down menu will come up. On my phone, there are a few choices of view contact, copy to dialing screen, send number, add to auto reject list. There are other options as create or update contact. I hope that helps. If not, google your phone's make/model and see if there is a link to instructions.
If you actually call the number, it gives you an option to put your number on a do-not-call list. In fact, it's the only thing you can do when you call the number, soooo.... why don't you try that?